When comparing Flutter vs Sciter, the Slant community recommends Flutter for most people. In the question“What are the best cross-platform GUI toolkits?” Flutter is ranked 15th while Sciter is ranked 19th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
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.
Pro Multi language
Support for C++, C#, Delphi, D, Go, Rust, Powerbuilder. See Go bindings on GitHub. The binding for C# on GitHub, SciterSharp does not seem to be free: in a commercial product you should acquire a commercial license.
Pro Lightweight
Only a single native DLL.
Cons
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.
Con Google has a bad history with product loyalty
Con Linux version is not very mature
The Linux version is missing HTML/CSS features when compared to the Windows version.
Con Not fully HTML5 compliant
Lacking HTML5 functionality and W3C standards: grabbing a library like JQuery or Bootstrap and use it in Sciter will not work.
Con Not WYSIWYG
Not WYSIWYG like WebForms or WPF.