That depends on your definition of "online experience". The 360 has the better UI, but the PS3 has had superior netcode in quite a few games.
It depends on the player base as a lot of games are p2p anyways.
For example, I've had numerous times where people said they had a better experience on PS3's netcode than 360's. Personally, I've always had a better experience on 360 - when not in a party chat, and met a few who agree but there are also others with opposite results.