Better than Mario Galaxy? i'm not so sure, it's extremely difficult to compare these games, they are like polar opposites.
that being said i think MGS4 is awesome and definately a must buy for any Ps3 owner, i think Mario Galaxy has the better sound (the music was absolutely incredible) but this game is no slouch.
gameplay styles are so different i can't compare them. So it is impossible for me to determine which is better. How about this...
MGS4 - best game on PS3
Mario Galaxy - best game on Wii.
Game of the generation: Both Mario galaxy AND MGS4 for different reasons.