When comparing IMO vs Threema, the Slant community recommends Threema for most people. In the question“What are the best messaging apps for iOS?” Threema is ranked 11th while IMO is ranked 20th. The most important reason people chose Threema is:
Registration is handled by random generated IDs. No phone number needed.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
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 Registration without phone number
Registration is handled by random generated IDs. No phone number needed.
Pro Encrypted communication
Provides complete end-to-end encryption.
Pro Company behind Threema is based in Switzerland
Pro Servers in Switzerland
Privacy laws protect the data Threema has.
Pro Supports encrypted group chats
Pro Provides a web client
You can control and message with friends via Threema Web, the official web client for Threema.
Pro Ability to send pics and videos
Threema supports sending media messages such as video, pics and even voice messages.
Pro Group chat polls
Pro Allows verification of contacts identity via QR Code
Cons
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 Not free
Unlike many other options Threema is not free as it costs $1.99 for the app.
Con Single device only
You cannot use multiple devices to send messages. However, it does have message mirroring on a web client.
Con No cloud sync
E2E encryption means no cloud syncing of messages.