When comparing Messenger vs Line, the Slant community recommends Messenger for most people. In the question“What are the best messaging apps for Android?” Messenger is ranked 21st while Line is ranked 29th. 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 Large celebrity and public figure usage rate
The caveat is that they are in Japan, but the app is commonly used to 'follow' public figures.
Pro Enormous, constantly updated stamp collection
Since many brands use stamps for promotional purposes, everything from movies to food brands to Taylor Swift have released stamp collections.
Pro Photo filter and editing capabilities
Users have in-app editing tools, as well as Line's separate camera application, which has additional tools.
Pro Integrated social network
Similar to Facebook's timeline.
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 Account is locked to the Phone Number & Device
The entire account is locked to the phone number, which cannot be changed. If the application is installed on a new device with the same phone number, the previous account data will be wiped.
Con Users have been concentrated in Japan
Of the 360+ million registered users, the top country is Japan (50 million). Thailand and Indonesia are 2nd and 3rd with 33 and 30 million, respectively, with very small market penetration in the US (10 million, 11th place).
