Ugh.
I voted GTA4 and just realized something ... GTA sales were on the back of a dominant ps2 and a fairly well established xbox.
By the end of the year ps3 sales will still be fairly low with xb360 leading by more than double. A few years from now that may change.
xb360 fanbase may surprise and buy H3 in equal numbers to GTA4, which would tip the scales to gta4 after adding ps3 sales. But I don't see it. GTA4 should be a great game, but the market has already been tapped a bit with Crackdown and Saints Row.
...after further review:
HALO3
if you are saying halo 3 > gta4 because gta is mainly bought by ps owners how do you know whether some people bought 360s for halo 3 or GTA IV??? especially when there is something that is exclusive to 360, you don't know.