Originally Posted By: -Jeran Korak-
If you must choose between the two, go for Halo: Reach. Please.
Black Ops is pathetic. The mountain of bugs and poorly balanced, massively unrealistic weapons seriously hurts the game.
It's totally not realistic to die when you get shot in the head with a sniper rifle, and most weapons have their own benefits, just because you might not be good with any other weapon which a lot of people are using, does not make it unbalanced. (Also, who cares about the campaign? It's a 5 hour long action movie)
Anyway, I would say they are very different games, with both pro's and cons. Halo is more fighting oriented, with vehicles, special upgrades and what not, while CoD is tactical, very little use of reflexes are needed if you can figure out what paths people take and how to go around the enemy team to kill them all from their backs.
Both are really fun though.