For the FUTURE, and I stress the future, Black-Ops is probably a viable option. For the present, it's CoD4, or, possibly nothing. The CoD4 server is empty almost all the time (Unfortunately. It was my favorite when it was filled.) and is really sitting there collecting dust and eating funds. I think what the problem was with CoD4 was that when BFM got into the mix, the game was on a downward spiral. Black-Ops, on the contrary, is up and coming. I would agree with Skedog, that when the prices go down Black-Ops will grow substantially. However, that isn't "now." Right now, Black-Ops probably isn't worth it. CoD4 isn't really worth it either.
I would go as far to suggest the CoD4 server be taken down if not active, and simply wait and see what happens to Black-Ops. If the game is worth getting into, then it's peak will be long enough for BFM to go in, set up, get familiar, and thus have it be worth the time and effort.
My little two-hundreth of a dollar.