If a stripper doesn't have a nice ass, I won't even think of having a dance with her, I might tip her, but that's it. In my 4 year strip clubbing career, I've only gotten dances from one girl that didn't have a nice ass (she has a model face though). She ended up becoming my ATF, go figure.
Tits on the other hand I don't really care about. As long as her tits aren't sagging or floppy it's fine with me. Anything from A cup to D cup is alright with me. Great looking tits are like a bonus to me, it's not required to get me to spend some money on her.
What about the rest of you?

Picking both would be a boring answer. You have to pick one and ONLY one.
For me, it's ass. I can't put into words what a nice ass looks like, I just know it when I see it.
Titties don't matter that much to me as long as the girl isn't flat chested.