Gears is very in-your-face graphically. I don't think there is a comparable analog in gaming for the third act. The mines and the rain and everything. I find a lot of revisionist history going on (not here), but in a lot of the media...kind of saying, "Yeah, Gears was amazing, but for these reasons...." when nobody really ever considered that previously.
Still, I think Halo 3 is very impressive and the art style appeals to me considerably more. Plus, we're talking about the MP here. SP is going to be better looking.