in terms of single player mode I heard the series nosedived after 3 and none are worth getting unless you play online multiplayer.
if you are going to download them. why dont you just download both. if you plan on getting one of those two then get black ops since zombies is pretty good, and people actually play MP
I can't recommend either unfortunately For some good DS FPS Action, go for Dementium (I and II), Moon, and Goldeneye is ok (don't flame me ) If you're dead set on COD, go for BO.
black ops! its not as good as 360, but its better than none. of you have black ops, add me: 154963025985