Here, here. I love my boobies more though. Can't help myself when I see a big ol' rack of tits. Though if they are lacking an ass it really does put a downer on the whole thing. It definitely is a pet dissapointment of mine, see a smoking hot girl walk past and when I look behind I see she has no ass. That sucks.
My logic:
There are unattractive/out of shape women with big breasts, but very few out of shape women have NICE asses (BIG maybe, but not NICE)
So I'd probably take a nice ass over a nice rack.