My PS3 is reserved for those few exclusive titles I can't get on 360. Everything else I get the 360 version. This is the case for a variety of reasons, not the least of which is that I enjoy getting acheivements. All other things being equal, I'd go 360 solely for the addition of some gamerscore to my total.
The trophy system was added to the PS3 so late, and the execution so poor that I can't be bothered to deal with it.
Largely, execution is what leads the rest of the decision. I prefer using my 360. The dashboard is better, the gamercads (profiles) are better, the online component is better, the avatars are better, the accessibility of downloadable content is better, even the controller is better.
There's also a case to be made that since the 360 is usually the native sku and the PS3 version of any multiplatform title a port that the 360 version often runs better.
All in all, the 360 fulfils my needs as a gamer more effectively than the PS3, and for that I'm more interested in playing my games on it.
-m