When comparing Messenger vs Allo, the Slant community recommends Messenger for most people. In the question“What are the best messaging apps for iOS?” Messenger is ranked 15th while Allo is ranked 30th. The most important reason people chose Messenger is:
Allows communicating with anyone who's a friend on Facebook.
Specs
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Pros

Pro Uses Facebook friends to create a contacts list
Allows communicating with anyone who's a friend on Facebook.
Pro Read receipts
Can see who has seen each message in group messages.
Pro Easy to share posts from Facebook
Integration with Facebook makes it easy to share any interesting posts from the Facebook timeline.
Pro Great UI
User interface of the application is clean, intuitive and easy to overview.
Pro Persists history
Your chat history is persisted by default (you can send incognito end-to-end encrypted self-destructing messages instead if you so desire).
Pro Allows self-destructing messages
You can send messages that will be removed from both parties after a defined period of time.
Pro Allows end-to-end encrypted incognito messages
Pro Assistant
The assistant is a bot that can help book, look up locations, restaurants, set up calendar events, etc.
Pro Full quality pictures
Cons
Con Lack of privacy
Messenger is owned by Facebook. Based on their privacy policy, this means that any information you provide (including messages sent) are used by Facebook to profile you and sell access to your data to advertisers.
Con Lack of end-to-end encryption
Secret (end-to-end encrypted) chats are only supported in one-to-one messages, and are not enabled by default, and the encryption has not been verified independently.
Con Not yet released (should release in summer 2016)
