In terms of campaign,
MW2.
it's storyline is more longer, better, and I think the graphics is nice on MW2.
While black ops storyline is a bit short, but nice as well.

In terms of Online.
Black ops.
Its more fun than MW2. And I think it has more dlcs than Mw2 aswell.

My pick would be black ops.
Due to online being fun as hell.