PS3, by a long way.
You can't compare it to GTA, because GTA is primarily aimed at an American market, and America is where the 360 is strongest. Also, the PS2 GTA games all ended up on Xbox, and did reasonably well there. Finally, GTA4 had two exclusive expansions for 360.
A large chunk of MGS' fanbase is in Japan, a place where the 360 version stands no chance. Perhaps 500k for the PS3 version, and 100k for the 360.
Others will be a pretty large victory for PS3- now-multiplat once-PS2 franchises always tend to sell better on PS3 in Others- even GTA 4 despite the reasons above.
America might be a slight lead for 360, and it might be a slight lead for PS3- it'll be close over there, only because of the 360's (relative to the PS3) massive userbase.
(Former) Lead Moderator and (Eternal) VGC Detective 
