When comparing iMessage vs Google Hangouts, the Slant community recommends Google Hangouts for most people. In the question“What are the best Skype alternatives?” Google Hangouts is ranked 9th while iMessage is ranked 17th. The most important reason people chose Google Hangouts is:
Hangouts offers both an Android and iOS app so users can participate from nearly any mobile device in addition to his Desktop.
Specs
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Pros
Pro Secure end-to-end encryption
Pro Cross-platform within Apple ecosystem
iMessage is available on iPhone, iPod, iPad, and Mac.
Pro Supports voice messages
Short voice recordings/messages can be quickly sent, and messages can be easily dictated into written messages as well.
Pro Free and unlimited messages over Wi-Fi and data
Messages clearly indicates whether messages are being sent over iMessage (blue) or SMS (green) where messaging rates apply. iMessages do not count as SMS messages with your carrier, instead they use a negligible amount of data to send over Wi-Fi and cellular data networks.
Pro Clear indicators for message delivery and receipt
The app indicates when messages have been sent, when they have been read, and when someone is typing.
Pro Comes pre-installed
No need to download or sign up for a new service.
Pro Seamlesss switching between SMS messages
Pro Friends don't need to "sign up"
You can just message them using their existing phone number or Apple ID and they will get your message.

Pro Clean UI
UI is minimalistic, and clear. And there is a plethora of stuff on the left of text box if you wanna use them.
Pro Apps and games for iMessage are available
For example, you can play games like pool or you can show someone a destination to go to.
Pro Mobile support
Hangouts offers both an Android and iOS app so users can participate from nearly any mobile device in addition to his Desktop.
Pro Screen sharing
Any user can share his screen (or only a specific desktop or window) while still continuing an audio conversation.
Pro Free group video support
Hangouts works with up to 10 participants using video out of the box or up to 15 if you are a business, government or school. It determines who is the primary speaker at any given time, and the camera view automatically switches to that user.
Pro Useful for businesses
Hangouts can scale to become a videoconferencing solution for an entire company, since the software can run on nearly all platforms. In fact, Google itself uses Hangouts as its meeting room video conference software.
Pro Easy to start using
Hangouts is integrated with Gmail & Google+, does not require you to download desktop software, and all chats can take place directly in your browser, significantly reducing the barrier to entry for a new user.
Pro Completely free
Google offers all features of Hangouts completely free. There are business, education and government solutions that increase the participant count to for video calls to 15 and offers support.
Pro Cross platform
Hangouts work on Android, iOS, Chrome OS, web and as a Chrome extension.
Pro Live broadcasting and instant recording
With Google+ “Hangouts on Air”, you can broadcast any hangout to an unlimited audience via your Goolge+ Profile or simply a generic YouTube link. Moreover, any Hangout on Air (public or private) is automatically saved as YouTube video that you can keep for private reference or use in any way that you would normally share a YouTube video. This allows you to record conversations without additional obtrusive software.
Pro Google Voice integration
Allows for a fixed phone number, receiving and making calls. Calling US or Canada is free.
Pro Integrates with Google Maps for sharing locations
When chatting with people while trying to plan a meet up, the share map button can be extremely useful.
Pro Android app does both sms and hangouts messaging
Cons

Con Not cross-platform
iMessage is not available on anything but Apple devices.
Con Can't send separate texts to individuals who share a Contact record
iMessage is an utter disaster. It can't send separate texts to anyone who is on a shared Contact record but with separate mobile phone numbers (e.g., "Paul and Angie Smith"). It merges ALL threads that ever went to ANYONE on that shared Contact record. Want to text just the husband to set up a surprise party for the wife? Not possible. iMessage will ONLY add that new text to the EXISTING thread you had with the wife. Duh! iMessage is pitifully incompetent.
Con Blocking Spam Messages takes 3 steps more than is needed
You have to open the message, select the sender, open info on the sender, then block sender. The default Android message app, select the message, block sender. it automatically deletes the message and blocks the sender.
Con iCloud sync of message is not stable
Sometimes, the messages on different devices with the same ID will be different, the syncing function work poorly.
Con Slow and unreliable
Con Poor video call quality
Only goes up to 720p30 + low bitrate.
Con Owned by Google
Con Lacking clients for older versions
Con High hardware usage
Con Not recommended for long sessions

Con Won't work on Windows 8-style UI
As Hangouts uses a browser plugin on the desktop and it doesn't work with Windows 8-style UI, it has to be used in desktop mode.
