-it has a ok campaign, but starts off boring at the beginning and becomes more interesting towards the middle and end.
-it has an OK online multiplayer.
-it also has more variety of guns than reach.
-zombies is more fun that infection (basicly Halo's equivalant to COD's zombies).
-it also has more challenging things to do like challenges and contracts.
- the campaign is good if you now a lot about halo, but if you never played a halo game beside reach you'll have to listen carefully to be able to grasp whats going on.
-the online is good and addictive (especially when you play with a party) as they added new things like assassination and armor abilities.
-fire fight is a lot of fun, because now you can get XP which goes towards your online rank.
-reach also has a wide variety of game types.
In my opinion, Reach wins it as it is less frustrating than Black Ops and the hit detection is better. Even though i prefer Reach i still like Black Ops and think it's a decent game.