I've always been a Playstation person, up until the PS3. Don't get me wrong, it's a great system, and doubles as a Blu-ray player. But there really aren't that many games I want to play for it, and playing online is nothing compared to playing online with an XBox 360 (though I suppose some of that depends on what game you're playing). I also much prefer achievements (360) over trophies (PS3).
The only thing (and this is a MAJOR thing) that I hate about the 360 is the fucking red ring of death. My ex and my 360 died from it, then the second replacement one died from it. My ex's brother's died from it. My boyfriend's died from it. All this means I have no 360 to play. And that pisses me off.
I have a PS3 to play, but I would much rather play a 360.