Given the user demographics, game genre and the difference in hardware penetration, it is unlikely (at best) that the PS3 version will outsell the X360 version. As others have stated, the Japanese market may make this more of a reality, but given attach rates there in comparison to other markets, I would suspect not.
GTA4, well, if I had a choice, I would buy it on the system that offered future content (360) so long as the other aspects of the original game were more or less equal.
Finally, the assertion that ALL PS2 users will magically migrate to PS3 (eventually if at all) is flawed at best. When someone is looking to "make the jump" - especially given that backward compatibility is no longer a given, what (beyond brand name) should convince me, as a PS2 user, to switch to PS3 over X360? They are both fine systems afterall...
For the record, I do own a PS3, AND any similarity between my handle and that of the OP is unintentional.
