When comparing Slack vs Telegram, the Slant community recommends Telegram for most people. In the question“What are the best messaging apps for Android?” Telegram is ranked 3rd while Slack is ranked 30th. 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 Drag & drop files in channels
You can upload a file to any channel over HTTPS simply by dragging and dropping.
Pro Fantastic search functionality
You can deep search messages, files and snippets. Given Slacks integration into many external services, Slack is good enough to act as the central search interface for your entire team.
Pro @mentions
You can ping people to get their attention even if they are not online by @mentioning them. Slack supports desktop notifications.
Pro Very polished user experience
The entire Slack interface is polished and intuitive to use. There are very few bugs or inconsistencies in the UI and it's very fast to use. There is nothing in particular that is new with Slacks implementation of team chat, but the execution of the groups (called channels), search, external service integration and notifications is close to perfect.
Pro Apps for iOS, Android, Windows Phone, Mac OS, Linux, and Windows
Slack has apps for iOS, Android, Windows Phone, Mac OS, Linux, and Windows to give you the full functionality of Slack with some extra features not found on the website on most major platforms.
Pro Supports multiple teams
You can be signed into multiple teams simultaneously and quickly switch between them.
Pro Freemium plan
Free forever, only restriction on searchable message archives, up to 10k of your team’s most recent messages and 10 apps or service integration. Great for trying out first.
Pro Robust integration with a huge number of tools
Slack integrates with tools like Trello, GitHub, Dropbox, Mailchimp, and dozens of others, so you can have a centralized event feed of your project right alongside your chat. This is tremendously useful for keeping context with your discussions.
Pro Emoji reactions to limit excessive posts and notifications
Pro Multiple channels for different groups
Pro Syntax coloring
Pasted code can be colored based on syntax.
Pro Edit messages easily
It allows you to change what you sent by hovering to the message and selecting "Edit message" under the ellipsis (...).
Pro GIFS
easily embed gifs with /giphy
slash command.
Pro IRC connectivity over SSL
Pro Flexible, granular notification settings
Notifications are handled separately for mobile and the web app. You can receive notifications for all messages, just direct messages, or based on filters, and you can have different settings for different channels: you don't have to get notified every time someone pushed to GitHub or every time someone posts to off-topic chat, unless you want to.
Pro Inline link previews (photos, mockups, etc.)
When a link is added, some content in the link is shown such as image - like how Facebook does it when you share a link.
Pro Slackbot extensible chat robot
The "Slackbot" can is an extensible robot that can be set you remind you about tasks, auto respond to certain phrases and a variety of other functionality.
Pro Dev team is invested, responsive, and friendly
Having submitted both feedback and support requests for bot development, I can personally attest that the team takes feedback seriously, and responds quickly to communication. This is vital for any closed-source or hosted project.
Pro Shows local time of each participant
You can click on the profile of a user to see their local time. An especially useful feature when members of your team are working in different timezones.
Pro Self chatting
Creative implementation which is very good for personal journaling or drafting/collecting ideas. Previously, to do this, users needed to create a private channel with themselves. Note that some other team-chat apps may disallow creating group/channel with no 2nd person(s).
Pro Can Create Diagrams with Creately
You can just create flowcharts, org chart, mind maps, UML charts, infographics and many more by typing /creately new <document name>
Pro It can surprise you
There's a checkbox in preferences under advanced options that may surprise you.
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 "Native" desktop apps are web apps
While it's great that Slack provides installable apps for Windows, macOS, and Linux, they're just the Slack web app wrapped in Electron shell. This means they don't offer the same level of native UX that a truly native toolkit app would.
Con Expensive when you need to upgrade
At $6.67 per user / month (or $8 if billed monthly) , Slack is significantly more expensive than the competition if you need features such as unlimited integrations (more than 10) or unlimited message storage (more than 10,000). However, the free version of Slack includes unlimited users.
However if you need only unlimited messages you can use storage services like https://slarck.com to upload then browse and search your entire message history, while staying in Slack's free plan. So with a combo of Slack+Slarck this con is not that major.
Con No self-hosting available
If you are worried about third-parties getting access to your data you should consider self-hosting. With self-hosting you are in control over where your data is stored, who has access to it. You will also not be vulnerable to exploits of a third-party provider.
Con Hidden max limit of free users per channel
Slack says that their free accounts support an unlimited number of users, which is true. However they don't mention that there is an undisclosed maximum number of users per channel (8462). For a large open source community, this is something to keep in mind.
Con Proprietary (non-free/libre)
Con Linux client is very RAM intensive
Con Slow and lags sometimes
Con No E2E encryption
Data is sent of SSL only, not E2E encrypted.
Con API doesn't allow custom widgets in chat
Con API is overall very poor
Can't do much with integrations.
Con Awful performance and constant glitches, since it is Chromium-based
You will experience a lot of hangs and glitches and it eats immense (for as basic as UI is) amount of RAM.
Con Replies are the worst feature implementation of all time
Con No dark theme
Con Cannot share screen through the browser client
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.