Mass Effect 2 is so much bigger and has the chance to be truly epic but the size can also become a weakness; there's more room for flaws.
I really can't decide which one will get the highest ranking but I think its fair to assume that both will beat the scores of the prequels.
The big difference for me though is that I know for a fact that I'll be playing ME2 but I don't know whether I want to play GOWIII since I generally find 3rd person action games really boring no matter how fast paced they are.
I don't know why but I never liked them, GOW, Ninja Gaiden, Heavenly Sword, Devil May Cry are all tedious games to me so they're obviously meant for someone else.