Currently, i develop for flash, not much ago i discovered unity, not yet played with it, but i have seen so far was cool.

Here are my thoughts:

Flash is more casual, start with cost less, in time and money. In unity you'd likely have to go more bussines-serious (real money).

There are proven bussines models in flash, like adver-gaming, ads, micro-transactions. Have not seen much movement in this in Unity, too soon maybe.

Flash is too heavy. By its nature(making games) Unity is way faster.

Flash is 2d, doing something 3d with it turns weird and slow. Unity is natively 3d, not optimized for 2d though, it is likely feasible as well.

I am overlooking the plug-in widespread, that gap will get closed over the time.