Both are coming out this year and both qualify as system sellers. Hyped sequels of very popular franchises. If GTA wouldn't have gone multiplatform, the result would be clear. 360 is going to eat away a lot of sales, that would have been PS3 sales, with the extra content and cheaper system. In the other hand, GTA has strong "Sony label". But which game will boost PS3 sales more? I's say MGS4 will, mostly because of Japan, unless PS3 sees a pricecut along with GTA4 launch.

MGS IV worldwide.
GTA IV in America.

 MGSIV actually might outsell GTAIV in America because of the Bundle, but I don't know if that counts.

GTAIV - MGS has had little success in the past (MGS3) compared to GTA - even only on one platform. I also sense more hype with GTA and I think GTA appeals to a wider demographic

GTA IS A FAR BIGGER GAME THAN MGS4

but with MGS4 being a bundle, it will definitely sell more PS3 hardware.

Just like how Dynasty Warrior 6 boosted PS3's hardware in Japan while DMC4 did nothing.

Hard to say DMC 4 did nothing because PS3 hardware severly dropped the next few weeks.

 Holy crap, psrock's mandate has been layed down! Fact: GTA is a far bigger game than MGS4

^ Sarcasm. No, I think there's a very hefty amount of PS3 owners who bought their consoles for at least the partial reason of owning MGS4.  

MGS4 should move more consoles cuz its exclusive and bundled. Also the open beta for MGO for us here in the US should help hype this game even more. Ofcourse if the MGO beta completely sucks balls then we will see a ton of negative press which will hurt sales.

I think GTA 4 will may sell more in North America but I think MGS 4 will sell more in Japan it may be close in the other areas.

@Aj_habfan: But i do think kingofwale is on the right track.
Besides, people who are going to buy a console just for GTA4 may see 360 Arcade as the best option.