Hard to tell, I'll try to share my cents though =>
PS3 for me ...I suppose? Nah, really just hear me out.
I figure its because most 360 games seem to be more popular over at US whereas I reside in Europe, (Scandanavia/Sweden to be more precise).
So yeah, it depends on games as well. Dedicated/Peer-to-peer, but in general, I would not say one service is smoother than the other. Live costs for gaming online and PSN is free. I still pay for Plus due to the insane amount of games but thats not what you want to know is it?
Moving on, we are at an age where technology is gold. Regardless of 5 million copies sold or 100k, you are bound to find players to play with, even in your region.
Are the devs good and the player pool rather big, you'll encounter a matchmaking that will automatically throw you into a pool with people that reside as close to you as possible.
As for the latency over US/Europe etc, it depends on how picky you are as well as what kind of game. Fight Night Round 4 apparantly spouses a lot of Europeans on the PS3 version so I am lucky to get that constant green bar whereas fighting US people is a pain in the ass and its choppy as fuck. The more you play the game, the more you can tell when its not "smooth" due to things feeling unresponsive etc. You cannot escape this for all time and a fighting game suffers far more than perhaps an FPS. (Fight night round 4 being a 2009 game which had a sequel sort of, but I hate it so I keep playing the old one :P).
In general though in terms of "smoother net", it comes down to your own bandwidth. None is really smoother than the other. Playing with your mate from next door on Live or PSN won't really matter. However the servers for each game can make it feel less smooth.
It comes down to the games netcode, where the players that are playing reside, if the server is hosted somewhere in Europe or US / is someone in the game hosting it in which you better hope he lives close to you and lastly how well is your bandwidth to recieve X amount of data or/and send Y amount of data to others.
