You need to actually play the games to know what they are like. Gameplay , Controls , etc.
Can't really tell much from the trailers , have to wait for them to come out . Reach comes out Earlier than Black Ops . Black Ops is coming out in November when Reach is coming out in Spring this year. (Australian Spring that is.)
Trailers are definitely not enough to base predictions on. You could get a slight insight into visuals and the storyline but like Holden said, gameplay, controls. SFX and music, interface etc. all remain mysteries.
From judgments based on my experience, I'd say that Black Ops is going to be better than Halo Reach. Like I say, we can't prefer a game based on a trailer. It's only when you get previews from professional critics closer to the release date and hands-ons that you can really start to deduce the quality of a game.
