I would also go with the PS3.
The online play is fine, and like Solid, I've only lagged once or twice, also only on LBP (Seems to be a problem with the games sync maybe, or loading all of the physics objects rather suddenly)
Take this whatever way you want, I don't really care, but I feel as though the PS3 is of higher quality then the Xbox. That's an opinion so I'm not getting into a fight about it. Find something better to do.