Well, hopefully someone who has used both can chime in too, but here goes(tl;dr warning ):
I know nothing about the Stencyl community. It may be great...or not. But C2 has a wonderful community. Actually, the quality of the C2 community is the main reason I choose to go with C2. If you have a question or problem, you can usually get an answer in a matter of hours, or even minutes. And Ashley (C2's lead dev) frequents the forum himself, and often replies to a dozen or more threads a day.
As to C2's capabilities, I think you can build pretty much any 2D game with it, short of, say, cortex command.
In terms of logic I think the way 'programming' is handled in C2 is really excellent. Essentially, you have a 3-tiered system:
Top Level: Object Types and Behaviors. With these you can literally build a complete game in a matter of hours, with just a handful of 'code'. Speaking of which...
Mid Level: Events. The C2 event system is an excellent compromise between flexibility and simplicity, and allows you to do almost anything. When you begin to feel constricted by what can be done with behaviors, this is where to head next. Of course, sometimes you hit a wall even here. So...
Low Level: Roll your Own (Object Types/Behaviors): C2 has an open plugin SDK, which the community has very much taken advantage of. There are TONS of plugins for C2, most are free, and the few that aren't are cheap (and your supporting an indie dev too ) Just take a look at the work of rexrainbow for starters (I think he's a vampire who doesn't sleep). Oh, and it's easy to develop for. I made my own plugin to add Mouse Lock(Pointer Lock) support with no prior javascript experience (though it did take a couple weeks on and off, but that was JS not C2).
Exporting and platforms...this is where things become a bit of a mixed bag. Exporting is in a far better state than ever before, but there are still issues to be sorted. For example, exporting to linux has broken for newer distros. Now, there are some ways around it, but they require some monkeying and are neither foolproof nor user-transparent. Of course...that sort of describes linux in a nutshell.
Mobile exporting is not perfect, but in the last ~6 months or so it's gone from dismal to pretty damn good. My knowledge here is a bit sketchy -- I'm still in the dev phase of my first major project, and I haven't dove deeply into exporting yet -- but my understanding is that Phonegap will offer perfect compatibility and great performance for any devices running IOS8+ and Android L (5)+. For all the devices running Android 4+, Crosswalk(based on chrominium) is shaping up to be a high performance and stable option, and is in active development by intel. OTTOMH, the people to ask more about this would probably be ArcadeEd, Silverforce, or cranberrygame.
Spriter is indeed awesome. Just thought I'd throw that in there.
Cheers, Tim