Because i don't want to be biased in my answer, i will let you choose. I'll give you the pros and cons of both of the games, and you can decide for yourself what you think is better.
Note that i have never played these games and am just making these statements off of. I will only make statements about what i can discern by looking at trailers, demos, etc.
We can start with Call of Duty: Black Ops (My preference, but that's just me)
Graphics
The graphics on MW2 were revered as the best of any 360 game, so Black Ops probably will be a step up. Almost every environment is different from the last, one missions you're in a Vietcong Tunnel, the next you're in a hovering Helicopter.
Anticipation
As much as i hate to admit it, Black Ops is only ranked #3 on the most anticipated games of 2010-2011, lagging behind Gears of War III, and Halo: Reach at the top.
Multiplayer
MW2 had one of the biggest multiplayer networks in the world, and Black Ops is supposed to be an expansion, so you can only imagine how big it's going to be. However, nobody likes Noob Tubers or Camping Snipers, both of which can get very annoying, and it is confirmed that grenade launchers will be in Black Ops.
Now for Halo: Reach.
Graphics
Though Halo 3 or ODST was never as revered as MW2 for graphics, it was still pretty frikin good if you ask me. The diverse environments and enemies added to the unique experience, and hopefully reach will be no different. However, some of the landscapes and characters are a little choppy, which has been a recurring trend in the Halo franchise.
Anticipation
As i stated previously, Halo: Reach is at the top of the food chain as the #1 most anticipated game of 2010-2011. However,the magazine which said this was Microsoft endorsed, so...
Multiplayer
The teasers and trailers have revealed little, but you can see the improvements all ready. The Legendary Map Packs on Halo 3 blew me away, so Halo: Reach will probably be above and beyond. Though there isn't as big a Halo network as there is CoD, its hard to tell which is better.
Well, that's it, I've laid out the good and bad of both games,