The MGS trailer did not show what the game will play like so it is hard to compare it to something that is being played like Gears of War.
And to be fair GoW looks as good if not better than MGS in certain aspects. Even looking at Chaos Theory on the original Xbox in screenshots and it actually looks better than MGS4 in terms of textures and lighting.
http://image.com.com/gamespot/images/2005/257/reviews/926596_20050916_screen006.jpghttp://image.com.com/gamespot/images/2004/screen0/921914_20041020_screen001.jpgIts not as high poly but its definitly comparable. And so is Gears of War, look at any close up pictures of the characters and you will see the same amount of detail as Snake.
Oh and as for co-op 360 games: These are confirmed for co-op play:
Gears of War, Kameo, PDZ, Saints Row