I saw gameplay videos for both on www.gametrailers.com, and in my opinion, without a doubt Assassins Creed looks to be the better game. And the main reason for this is the combat. AC looks like it has awesome combat, with plenty of stealth also. And AC's combat is in real-time, unlike ME's. The turn based (pause-play-pause-play) combat of ME looks like it would get really old, really fast. And I don't believe turn-based combat is needed for RPGs, just look at Oblivion, it's one of the best RPGs ever made and it's not turn-based.
Both games look to have awesome graphics, but the gameplay of AC just looks better, IMO. I think ME is only being hyped more here on System Wars because it's an exclusive, and that's a silly reason to hype a game. BTW, I own a 360.