I owned an xbox for a fair while mainly because;
At the time it was cheaper.
Better games for me(Halo, GOW)
Controller is a lot more comfy imo
All my friends owned one.
If I had the choice between the two, like they were the same price, I'd probably choose the PS3, simply because I hate paying for online, I can get over the loss of some games and a good controller to save $90 a year xD.
Sold my xbox about 2weeks ago though, PC>all. ;D