When comparing Stencyl vs Phaser, the Slant community recommends Phaser for most people. In the question“What are the best game engines for web games?” Phaser is ranked 2nd while Stencyl is ranked 8th. The most important reason people chose Phaser is:
Phaser keeps things simple and as such is easy to use by beginners.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Haxe scripting available for advanced users
Power users can also write code in Haxe (similar to Actionscript 3) to create their own custom classes and extend the engine.

Pro No coding required, great drag & drop interface
Visual scripting in Stencyl is based on the MIT Scratch project, which was designed to teach programming. Script elements fit together like puzzle pieces, ensuring that data and function types cannot be mismatched.
Pro Cross-Platform
Publish iOS, Android, Flash, Windows and Mac games without code.
Pro It's a NO-CODE Program, you can add logic without code
Pro The original concept for Ghost Song was created using Stencyl
The original concept for Ghost Song was created using Stencyl 3.x
Pro Great performance on every platform
Stencyl exports your games to native code so they have great performance on every platform.
Pro Beginner-friendly
Phaser keeps things simple and as such is easy to use by beginners.
Pro 1000s of examples
Thousands of example are on the Phaser website, which show everything you could want to do with Phaser.
Pro Supports WebGL with canvas fallback
If WebGL is unavailable, Phaser automatically switches to HTML5 canvas.
Pro Targets mobile browsers
Built specifically for mobile web browsers.
Cons
Con Not a powerful engine
Should be used for basic games only.

Con Only available via subscription
There should be an option to buy it outright, especially considering it is written by a one man team....this is not exactly an Adobe level enterprise with shareholders, so there is no excuse!
Con Slow release cycle
Con Updated
It needs much to improve for mobile games, it was left in the era of Flash games. In Android you can not even put the native keyboard, you can not access things like native camera, GPS or native text input.
Con Tile system is somewhat inflexible
Con Missing accessibility features
While not a big issue, it may be a dealbreaker for some.
Con Poor code structure
There's little in terms of cohesiveness in classes, methods or patterns.
