Personally I think both of those games are great, and not only that, they each have a great community to back it up. It's too hard to say which one's better, so I'll just say it's a tie for me.
Honestly GOW2 is only good for multiplayer in my opinion. The single player gameplay in GOW2 and 1 can not compete with the MGS series (any of them) but I'm not a fan of the multiplayer in MGS. Which bring me to my official response of MGS4 because plenty of games have wonderful multiplayer but finding a good single player game is a much harder task.