When comparing Hangouts vs Telegram, the Slant community recommends Telegram for most people. In the question“What is the best team chat software?” Telegram is ranked 3rd while Hangouts is ranked 15th. The most important reason people chose Telegram is:
Copy/paste videos, gifs, images, geolocation, documents, and even executable files. Drag over files for practically any time up to 2GB in size. Images can be compressed, too.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Save and sync your chats across multiple devices
Start chatting on your phone, then on your computer, then on your tablet seamlessly!
Pro No need to create a separate account if you use any other Google service
Hangouts uses Google account credentials. If you use any other Google service, then you already have an account set up.

Pro Group video support
If you have signed in your Google account, Hangouts allows making video calls with up to 10 people or one-on-one. There are business, education and government solutions that increase the participant count to for video calls to 15 as well as offers support for those accounts.
Additionally, you can switch between using either the front-facing or rear-facing camera on your phone or tablet.

Pro Free and cross-platform
Google offers all features of Hangouts completely free on iOS, Android and web with quick and reliable sync across all platforms. It's also tightly integrated in other Google services - it can be accessed from services such as Gmail, Google+, Chrome.
While the standard Hangouts offering is completely free, there are business, education and government solutions that increase the participant count to for video calls to 15 and offers support that may cost money.
Pro Chrome browser extension
Use the extension on any computer you can run Chrome browser to text from your computer or tablet!

Pro Excellent read receipts in group chat
It's possible to know what each and every participant of the group has read. Their icons are displayed below the last message.

Pro Can share animated GIFs
If you have gifs in your mobile phone, you can share them in the conversation and your partner will be able to see it animated in the chat.
Pro Google Voice integration
Google Voice is a service that enables making and receiving free phone calls within US. Google Voice forwards incoming calls to all connected devices and allows placing calls from any of the connected devices, but they will show as coming from the Google Voice number.
Hangouts completely integrates Google Voice, allowing you to place calls and receive texts and voicemail from all of your devices. The Hangouts app for Android also includes a needed dialer for making calls with the users Google Voice number.
Pro Free calls to US and Canada from anywhere in the world
Hangouts app for Android can be extended with an optional dialer for making calls. Most calls to US and Canada are free and international calls have cheap rates.
A Google Voice account will be needed in order have a number that allows these calls to be made.
Pro Advanced status sharing
Hangouts can inform about the status of other users including what device they are currently using, whether they are in the middle of a call and when were they last seen. This allows making sure that you're not disturbing someone when initiating a conversation. It's possible to granularly turn off status sharing.
Pro Video tools
Hangouts can add different effects and overlays to video calls that can be viewed on your device. Unfortunately, they can't be set from your phone.
Pro Seamless integration with OK Google for voice control and typing
Hangouts can be used completely hands free, even if the app is not open and the phone is locked: "OK Google, send a Hangouts message to Mary".
Pro Free and convenient large file sharing
Copy/paste videos, gifs, images, geolocation, documents, and even executable files. Drag over files for practically any time up to 2GB in size. Images can be compressed, too.
Pro Free of cost
The application is available free of cost.
Pro Privacy focused
In addition to the end-to-end encryption [only in secret chats] that provides privacy to communications, the app also includes self destructing timers for chats, and the ability to permanently delete messages, yours and theirs, from your and their devices.
Pro Media messaging
Videos, gifs, images, geolocation, and documents.
Pro Channels for one way broadcasting
Broadcasting channels suitable for announcing things.
Pro Beautiful UI
Very customizable with a lot of clean looking options.
Pro Possible to send photos and videos without compression
Pro Supergroups of up to 200,000 people
In Supergroups, anyone along with admins can delete their messages from all sides. A message can be pinned to the top. All of the user's messages can be deleted by an administrator, and they can be banned and reported as well. Supergroups can have a public link (outside of Asia, due to high spam volumes), which is also a username.
Pro Two-step verification
You can set up an additional password to your account, which can be reset by your email. You don't have to use an email if you do not want to. If you lose access to your email or forget your password, you will be able to reset it via text message if your account is over seven days old and will receive it within a week (in case you recover anything before then).
Pro Secure
Pro Saved messages
You can write messages to yourself. You can write some notes, add links, and see everything across all your devices.
Pro Cloud chat keys are kept in different jurisdiction
Even though encryption used for cloud-based storage (e.g. Gmail, Facebook, iCloud) may not be as secure as end-to-end encryption or zero-knowledge storage, Telegram does the most that it can by holding keys to chats in different jurisdictions, requiring intergovernmental cooperation to obtain the keys. This is aside from whatever other security measures have been taken, of course.
Pro Multiplatform native client apps
There are native desktop, mobile and web client apps, that are optimized for speed and are light on memory usage.
Pro Open source client apps
Client apps are open source, making whole service more trustworthy.
Pro Unlimited storage
Unlimited number of files can be stored forever on the cloud. You can upload files up to 1.5 GB each.
Pro Voice and video messages
You can post voice and video messages if you are too lazy to write text messages.
Pro Group Voice Chat
This Discord-like feature adds to the interactivity of the application.
Pro Pinned Messages
You can pin messages to the top of groups or supergroups so members will see them when opening the group.
Pro Self-destruct messages
Messages in individual chats can be set to self destruct on a timer. They will disappear after the timer expires.
Messages in all new chats can be set to self-destruct with a global auto-delete timer in Settings > Privacy and Security > Auto-Delete Messages.
Pro Edit or delete messages after sending them
Pro Banned in China
Pro Save time with inline bots and link previews
Unfortunately, on the desktop videos don't play, but images are viewable without visiting popular websites like imgur and Instagram.
You can play YouTube videos and others within the app, so you don't need to embed code or anything.
With inline bots, such as @gif, @pic/bing, @vid/YouTube, etc, you can look things up without leaving the client or chat that you're in. Simply type the name and some keywords and wait for the results to load.
Pro @mentions
You can ping people to get their attention by @mentioning them.
Pro Desktop app handles HiDPI
The desktop app has a built in setting to scale the UI of the app up to 200% which allows for those with 4k monitors/screens to use the app without scaling issues.
Pro High quality calls
Uses the latest GPL audio codecs, such as Opus, an adaptative high-quality codec that supersedes AAC at any bitrate.
Pro Customer support is within the app
Although it's volunteer based, but still pretty top-notch if available, which it tends to be.
Pro Large groups can have channels called 'Topics'
Topics are now available in groups of 100+ members and have a sleek two-column layout and new buttons to quickly navigate.
A default topic called General is now in every topic group, holding the group’s service messages and previous message history.
Pro Popular in the USA
On SimilarWeb you can see that Telegram is on the top 15 among all communication apps.
Cons
Con Transitioning to corporate orientation
Google is turning Hangouts in to a Slack competitor. It's unclear what the future of Hangouts is, but Google has said the consumer Hangouts we now have 'isn't going anywhere.'
Con Lack of end to end encryption
Communications are encrypted between client and Google servers but not from sending client to receiving client. This means that there is still a possibility for someone other than the receiver to read your message if they have access to internal Google servers. But it allows also lightweight chat history search, spam protection and other features.
Con Is owned by Google
Google is a cooperation that earns money from collecting data and breaching privacy.
Con Poor interface
The app tends to lag, the design can be confusing and considering that another app has to be installed that supplies another icon in the home screen in order to get a dialer for calls in Hangouts, overall it is very confusing and cluttered.
Con No rich text editing
On Android you can receive messages with bold or italic text. However, you cannot bold or italicize text on Android, only on iOS devices or via the web interface.
Con No way to moderate groups
If a member of a group chat room becomes disruptive, or there's some other need, there's no way in Hangouts to remove somebody from an existing chatroom. Similarly, if somebody is no longer active, there's no way to remove that user from the chat room to make slot for new active members.
Con Buggy video calls
Whether video calls work on your device is pure lottery, and there's nothing you can do to fix it.
Con Poor indication of who's online
There is no way to find out if a person in your contact's list is online or not in the Android app.

Con Poorly manages data
Doesn't cache images well either received or sent. Often users will see the sent images re-downloading, same for conversations.
Con API support was removed
This change closed options for integrations and connections between similar apps.

Con Not available on Windows Phones
There is no Hangouts app for the Windows Phone.
Con Doesn't support sending voice notes or video recordings
Currently there is no built in function that supports creating and sending voice notes or video recordings.
Con No customer support
Free product, you're on your own.
Con Won't send SMS/MMS anymore
They removed support for sms/mms. The same goes with integration to hangouts.
Con No SMS/MMS backup
They don't have options to backup sms to cloud.
Con SMS messages sent from the web app can't be sent outside North America
SMS messages sent from the web app can only be sent to number in America and Canada - sending to foreign numbers won't work, although you can still receive texts from anywhere.
Con Not fully open source
Telegram has an open API and protocol free for everyone, but the backend software is not open source. Therefore 100% privacy can not be evaluated.
Con Can only be used with a phone number
It is not possible to activate and use Telegram by mail etc. You need a phone number.
Con Only "Secret Chat" messages are encrypted end to end
The only way to get end-to-end encryption (client-client) using Telegram is to use the Secret Chat function, anything through the service is server-client encrypted. Also secret chat is only implemented in the mobile apps, meaning there is no way to encrypt messages end-to-end when using the desktop client or the web browser client.
Con "Secret Chat" feature is not available for desktop client
Secret chats with end to end encryption are only available on mobile apps.
Con No source code sintax highlighting
Telegram does not have source code syntax highlighting, making it difficult to share code snippets.
Con Does not work for visually impaired
Doesn't work for visually impaired.
Con Does not support spherical 360° photos
Spherical 360° photos becoming more popular these days, but Telegram does not support such format.
Con Does not support text messaging
You can NOT make it your default text messaging App.
