I hope GTA IV will do well for both systems, with regard to which version will sell better I think the PS3 has a higher percentage of GTA fans, but the 360 has a larger installbase and this is IMO really by far the 360's most important game for this year. So IMO it could be about equal.
More so than driving 360 vs PS3 sales, I think it will be important with regard to driving high definition gaming (360+PS3) against Wii/PS2 sales, at least an important reason for many PS2 gamers to upgrade.