Flash Game, erase or add

Yoyo, allright I’m working on a big project right now and I’m a bit puzzled about the logic of the coding.

Basically you have 2 bitmaps, and you erase parts of the top to reveal the bottom.

I could achieve the same effect by drawing the bottom bitmap over the top bitmap.

So, either I erase parts of the top. Or I paint parts of the bottom on the top, making the illusion i “erase”.

Which would be best, coding-wise, performance-wise etc. which would give me the most flexibility in terms of “seeing where i erased”, hitTests etc.