MW2 was an amazing game. The Campaign was a little short for some people, but then you move on to the online gaming and you might not get bored of that for months. If you wanted a change of pace, you could either play local matches with friends, or special ops with friends online or locally. I love the idea of the special ops because it was almost like having a second game to play/conquer. I've spent the past year trying to get all the stars on every mission, and it has taken me almost all year to do it! I am also a trophy hog, so I love that you continuously earn trophies as you complete the special operations.
I wish they would have included some spec ops missions for black ops. I think the game, so far, is every bit as good as MW2 as far as storyline, game play, graphics, audio, etc. The online gaming is pretty cool, with new maps, different weapons, and innovative killstreaks ( I love that you can get attack dogs!). Also, you have to "purchase" your weapons, challenges, and contracts.
I think the zombie mini-game is going to be fun, just like it was with COD World at War, but there's no way it's going to be as interesting, challenging, and addicting as the spec ops in MW2.
While I think the game is awesome, so far, I could see myself feeling like "I've done all I can do now" a lot quicker than I did with MW2.
My recommendation? Though I think MW2 is a larger game with more options, I would go ahead and get Black Ops so you can play/learn it while everyone else is learning it. That way, you won't fall too far behind when you're playing online. In the meantime, play MW2 at your friend's house.
MW2 will probably be dropping in price pretty soon, so you may be able to grab a new copy for 40 instead of 60. But, if not, there are going to be alot of people selling MW2 so they can get black ops. You can check on ebay, craigslist, etc... and probably get it pretty cheap.
Good luck man. You can't go wrong with either game, no matter what you choose. Have fun.