While both had twisted plots, I have to say Modern Warfare 2's was better due to the fact the the Task Force 141 was always getting betrayed. Modern Warfare 2 also had a fast-paced plot. Black Ops however, had a more revengful campaign.
Modern Warfare 2 had a more easy-to-use create a class setup while Black Ops was more confusing. Black ops however had less levels which was good but was canceled out by the extra 5 levels of prestige.
Both were enjoyable but although Zombies was fun, it got repetitive. Spec Ops had more of a variety of missions to choose from along with regular, hardended and veteran.
You decide which is better. Leave me a comment telling me which one you think is better.
This is completely opinionated and should not be hated on.