I think nearly everything important has already been mentioned. I've worked on a few small mobile projects in Stencyl and it's quite fun to use. Unfortunately the whole thing feels really dead, there's still some development going on but it's slow as hell and there's nothing happening on the forums.
The best thing I can say about it is that the way you build logic is the closest thing to real programming I've encountered and it's also really good at teaching you a lot of principles of real coding. Because of this learning takes a bit longer and progress feels slightly slower in the beginning but over time you'll get better at building reusable logic.
So, here's a quick subjective comparison:
o Stencyl is fun to use - Construct 3 is more fun to use.
o Learning and building stuff with Stencyl takes more time than with Construct 3
o The way you build logic in Stencyl feels a bit closer to real programming than in Construct 3
o Stencyl's development is worryingly slow - Construct 3's development is super fast and the devs are insanely active
o Stencyl has native export - Construct 3 doesn't
o Testing your project in Stencyl and the software in general is slower than Construct 3
o Construct 3 is web based and can be used on a lot of devices with a browser - Stencyl can only be used on Mac and PC
o Stencyl's forums feel close to dead - Construct's forums feel much more lively