Hard to say, the 360 has a much bigger hardware lead in america now, which is gta's biggest market. However, this late in the gen it is hard to say how many of those systems are still in active use...
Honestly, it will probably come down to two factors as to who wins.
The first factor is when the next gen systems come out. Xbox and ps3 software sells will probably start drop once the next gen is announced and drop a ton when the systems come out. If the nextbox hits a long time before ps4, it will help the ps3 make up the ground from an inevitably stronger 360 launch.
The other big factor will be marketing and possible exclusivity deals. Microsoft has shown a willingness earlier in the gen to buy timed exclusivity, but their strategy seems to have shifted now. I think a more likely scenario would be the ps3 getting some kind of timed exclusive deal. Either with dlc or the game itself. Rockstar seems to prefer sony, and it might be a good way to get out of their exclusive deal with agent, which appears to be vapor...
I would say the conditions favor sony this time, but hardware advantage still belongs to microsoft...







