When comparing Messenger vs Atlas, the Slant community recommends Messenger for most people. In the question“What are the best Material Design apps?” Messenger is ranked 22nd while Atlas is ranked 71st. The most important reason people chose Messenger is:
Allows communicating with anyone who's a friend on Facebook.
Specs
Ranked in these QuestionsQuestion Ranking
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 Built-in ad blocking
The browser uses EasyList data to remove ads from sites. It can further be supplemented with EasyPrivacy data to limit website ability to track you.
Pro Stores preferences on per-site basis
The browser can be set to remember which sites need to be shown in desktop mode, with certain content filtering settings, etc.
Pro Can browse two tabs at once
Atlas has a dual pane view that shows two pages side by side.
Pro Can be set to very low data usage
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 Abandoned
Atlas was last updated on September 19, 2014.
