When comparing Avalonia vs Flutter, the Slant community recommends Avalonia for most people. In the question“What are the best cross-platform GUI toolkits?” Avalonia is ranked 3rd while Flutter is ranked 15th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Open source
Pro UI designer plugin
A User Interface designer plugin for Visual Studio can be downloaded from the Visual Studio Marketplace.
Pro Open source
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro A single codebase for iOS AND Android
Pro Fast
The developer's goal is to allow people to make apps running at 120 FPS.
Cons
Con Beta
Still in beta status.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers. BUT if you have to learn it ... it's a con.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.