In Europe X360 owners will chose GTAIV for sure. In USA Halo 3 is secure. X360 means more in US so overall Halo 3 will keep the crown. Yeah, it's as simply as that.

Halo 3 most likely

I would say that they will be quite close, but Halo 3 will still win. Currently it is at about 6m, I don't think GTA IV could do more than that lifetime, although I do expect very high PS3 sales for it (close ish to 360).

I think GTA, ofc not right away but it will have longer legs. GTA San Andreas sold some 20 million copies according to a member the 'upper' taketwo heriarchy. I plan to get a 360 just for GTA. Personally, imo Halo is way overhyped, remove the online and no one would even care. Crysis, Metroid Prime 3 & GoW are better in different ways and I'm sure there's more. Dont flame me tho guys I never said I didn't like Halo!

I think GTA will sell 8m on X360 !!
and Halo 3 will sell 11m ..

 PC. Even though it may not be a synchronized release, all GTA games are on the PC as well, usually a year or so after consoles.

In Europe GTA4 will kill all of the compitition because it is a huge franchise there. Also, Yes it will sell more on the PS3 there then it will on the 360.

In Japan (assuming it gets there) GTA4 will sell more. Most of the GTA games in Japan sell roughly around 300,000 copy's but it helps that Capcom publishes it over there.

In America it will be very close. On the 360, Halo 3 will sell more no doubt but overall I think that GTA4 will sell the most counting PS3 sales. Halo 3's big selling point is in America where GTA's is in Europe and America.

Overall: I think Halo 3 will sell more copy's on the 360 but overall counting the PS3 worldwide GTA4 will sell more then Halo 3. Im expecting Halo 3 to end its sales with around 7 million world wide. Im expecting GTA4 to stop selling with around 12 million sold worldwide. Just remember GTA: Vice City is still in the Wal-Mart sales case for games and most games dont even last 1 year before its replaced with better selling games.

I'm sad to say that Halo 3 will probably marginally outsell GTA4 on the 360, but GTA4 will cream H3 in overall sales. The attach rate of past franchise entries is not really a valid metric in this case. There were a lot of reasons to buy a PS2, but Halo was pretty much the only reason to buy an xbox (except maybe for graphics whores with an aversion to PCs). This time around it is a different story. The real litmus test will be how many 360s are sold on the back of GTA4. Both consoles will definitely see a sales spike.

GTA IV FTW

GTA IV, not only Europe, even USA