360 will sell more copies of the game.
However, I think the game will benefit PS3 console sales more.
Why? I think that majority of people who would want to play this game on a 360 will have one, so the game will sell well on the platform but only drive modest spike in consoles. I do not think that majority of people who would go with PS3 have bought yet, therefore I expect to see a better spike in consoles for the platform as GTAIV combined with BR and better library convinces people to make a choice.
I'm sure MS knows this as well, and that is why it is going to go crazy with the marketing and paid for the exclusive DLC as their plan must be to try and reduce as much as possible the spike for PS3 and hopefully convince some prospective PS3 purchasers to go with the 360 instead.
The DLC doesn't matter if you have a console already. There can't be many PS3 owners who would buy a 360 just to get a few extra episodes (there may be some, but not enough to matter in the big picture). The DLC is all about something extra to attract fence sitters.
Try to be reasonable... its easier than you think...
