When comparing IMO vs GroupMe, the Slant community recommends IMO for most people. In the question“What is the best team chat software?” IMO is ranked 20th while GroupMe is ranked 49th.
Ranked in these QuestionsQuestion Ranking
Pro Video call support
Pro Beautiful User Interface
Although IMO has a comparable, and in some cases superior, feature set when compared to the competition its key selling point is its interface.
From LifeHacker when it won "Best IM Client":
Signing on, navigating chats by tapping the tabs at the bottom, searching for buddies, and virtually any other operation is easy to find and use. The app, itself, is very quick. Even sending a voice IM, if you don't feel like typing, is responsive and sends quickly. Everything imo does it does well, but it's excellent navigation really makes it top notch.
IMO also has a fully functional website that you can use for chat while on your desktop or laptop. This really puts IMO ahead of the competition as you can use the same system on all your devices so you don't have to deal with overlapping notifications etc. (Note: IM+ also has this feature)
It should also be noted that it has the highest rating in the App Store due to both the UI and its stability.
Pro Easy to switch between conversations with tabs
Tabbed chats make it easy to switch between conversations
Pro Lists and chat histories are searchable
Both your buddy lists and chat histories are searchable.
Pro Accounts can be linked so you don't have to sign in to several accounts
Pro Push notifications
Pro Concurrent sessions
Concurrent sessions allow you to be logged in on your desktop while also being logged in on your phone.
Pro Chat groups
Pro Reads receipts
GroupMe has a built in function to read receipts.
Pro Great web client
GroupMe has an excellent web client that can be used to send and receive messages and manage groups from the desktop.
Pro IFTTT Integration
Allows sending messages from web services via IFTTT.
Pro Integrated calendar
It's possible to add location and time with reminders directly in the GroupMe app.
Pro Makes it easy to silence 1 or all groups
GroupMe has settings that allow muting a single group or turn off all GroupMe notifications from within the app.
Pro Does not require everyone in the group to have a smartphone
GroupMe can set up a custom phone number which allows people without the App to send and receive messages. This makes it easy to add anyone to messaging.
Con No support for third-party instant messaging networks
On March 3, 2014, IMO will start discontinuing support for all third-party instant messaging networks.
Con Drains battery fast
GroupMe, in order to have a constant connection which allows for notifications drains the battery of a device quickly, which is not ideal for a messaging app that one needs on time notifications from.