I never understand why threads like this come out where the two game are so different. It's mind boggling.
Brink is a science fiction FPS heavily based on story and customization. It has a semi-cartoony look to characters and environments, and does not have the typical multiplayer aspect to other FPS games. When you play against other players in Brink, you are actually playing a piece of the campaign with the AI enemies replaced with real people. Another real emphasized part of the game, although not much of a forced purchase incentive in my opinion, is the SMART button. Hold down whatever button it will be, and your character will traverse random terrain similar to the way you would in Mirror's Edge if you've ever played or seen that.
Black-Ops is another modern Call of Duty that is very much the same, just with a different developer with new ideas. If you've played and liked MW or MW2, chances are you'll like BO. Lol, "BO." Differences with BO is that instead of unlocking weapons in like in MW2, you'll actually accumulate credits while playing that you will spend on weapons of your choice. There will also be an emblem editor that will replace the player bars, and you'll also be able to do other little tweaks like have your clan's name on your gun, view, edit, and upload videos of games you played just like Halo is famous for, a favorite for me is that now shotguns aren't secondaries! Yay!
But like already stated, Brink does not have a release date, just a window. You'll likely be able to acquire the money for both if you set your mind to it.