If you want both, I'd grab Black Ops first. Chances are, Black Ops is going to retain its price through next year just like every other Call of Duty. Assassin's Creed 2 had dropped in price after a few months of being released last year, so you should be able to purchase it cheaper much quicker than Black Ops.
My personal choice would be Assassin's Creed Brotherhood as it is one of my favorite series ever, but the cheapass in me is telling me that grabbing Black Ops now and Brotherhood later down the road is the best option.