First of, people (PS3 fans) keep saying that when MGS4 and FFXIII come out PS3 will triumph...
1) Mgs isn't that big of a franchise in Japan, which is already Nintendo Pacman.
2) FFXIII has BIG 1st week sales combined with good hardware sales for the system it is on (PS3).
3) Mgs 4 can't sell more then 500k in its launchweek in Europe and America because of the PS3's userbase!
Honestely, i don't expect it to do around 350k in America first week and 170k in Europe. Mario Galaxy managed 600k USA and a little over 200k for Europe... And that is CERTAINLY a game that will end up with the higher attach-ratio. As such, it will have better legs and better launchsales in Europe, America AND Japan!
Mgs will be a millionseller but i think it will follow around the same path (total sales wise) of Metroid Prime 3.