I forgot to say what I would do...
If the game has major stability issues (framerate, loading, etc.), I'll go with the stable one.
If not, I'll go with the Playstation.
For some reason, I prefer the Trophies over the Achievements.
Even for multiplayer games, I'd stick to my PS3.
Most of my friends aren't hard core gamers and bought the PS3, because the extra content (blueray, music and movie playbay, etc.) offered by the console appealed more to them.
However, the Xbox is still being used a lot for exclusives!