To be completely honest with you... I've been waiting for a way to do paper doll 2D animation without having to write my own tools for a long time. To me it's a matter of what is most convenient and more well supported. Spine's editor is more complete, it looks polished and very usable. But they only support Corona and don't have a publicly available file specification yet.
I have an interest in sticking with Spriter, I already bought a copy of the software, and there are available Actionscript APIs that implement the file specification (I work with Haxe, so I need to do some porting anyways, but this is a significant plus).
The way I see it, Brashmonkey has 16 days to put out a big resounding reason why I should stick with Spriter. Otherwise I'll back the Spine Kickstarter before it ends, and postpone deciding which tool to stick with for my current project until they both mature a little.