There is a slight performance with AIR interms of graphics being moved around very fast. This should go away once stage3D is available in the runtime. I am not too certain if most users would notice. I have only played a little with Cascades, but coding seems a little faster in AIR and it is a light more robost developerment environment. Cascades is still evolving, so some of the little things are still being worked out. However, it is probably best to test both environments. Cascades has a lot of working samples so you can probably just look at the code generated in those samples to give you a better feel.