When comparing Onsen UI vs Flutter, the Slant community recommends Flutter for most people. In the question“What are the best frameworks for developing cross-platform mobile apps?” Flutter is ranked 1st while Onsen UI is ranked 10th.
Ranked in these QuestionsQuestion Ranking
Pro Open source and free
Onsen UI is open source, and completely free to use.
Pro Can be used for web apps as well
Onsen UI is usable for classic web development as well as mobile.
Pro Large selection of components
Onsen UI provides a wide selection of components that are ready to use out of the box and are easily customizable.
Pro Easy to use
Onsen UI provides very clear, well-written and detailed documentation.
It uses popular technologies that are likely already familiar to developers (such as jQuery and Angular).
The framework is semantic and intuitive to use, making it quite fast to learn.
Pro Supports React
React is the most trending front-end UI framework, naturally supporting that is a plus.
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 Open source
Pro A single codebase for iOS AND Android
The developer's goal is to allow people to make apps running at 120 FPS.
Con Still in development
According to the website, Flutter is still in its early stages of development.
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.