It basically comes down to whether you want html5(c2), or exe (classic).
c2 is also more stable, while cc you have to save more often in case of a crash, and occasionally you'll find a bug that you have to work around. Don't get me wrong though, its just quirky, not completely broken, and the games it makes are stable and run superfast, and with that extra speed comes the ability to make much more graphically complex games. Add to that pixel shaders and sprite distortion, and cc has the ability to do the gfx from any 2d aaa title given the right artist
Also, as far as documentation, most of what you learn in c2 can be used in cc. The logic is the same, and after learning one, you don't have to really learn the other. There are small differences, like in c2 solid is a behavior, cc still has behaviors, but for things like solid, that don't involve movement, its a property.
c2 for the streamlined editor/ide, and the multiplatform webbased html5, or cc for the quirkier construct editor, with more plugins, and a windows only, but more powerful runtime.