My own view - which will come as no surprise to you lot - is that MW2 is by far the superior game. Treyarch's attempts to bring balance to Black Ops actually resulted in a homogenous mess where all guns feel pretty much the same, with the same attributes. The ability to buy your attachments and perks, while initially seeming like a positive step, actually detracts from your progression as the same incentive is not there to prestige and to unlock all the various weapons. This in turn takes a way a lot of that urge to keep playing that was prevalent in the other CODs. There are various other issues I have with it but this post will end up a novel, so I'll leave it there.
Aside from the playability tweaks which in general can be considered subjective preferences, the one thing that is utterly unforgivable about Black Ops is the awful netcode. Winning or losing games more often than not hinges not on player skill or team capability, but on the connection to the host and how good or poor it is. This means you can play in the exact same lobby and in one game win 7500-2500, and in the other the score can be reversed in the other team's favour. This is not "balance", it destroys the idea that your fate is in your hands, and is instead down to how well you happen to connect to the host. Now I know this is the case to an extent in all online multiplayer games, but in the case of twitch shooters like COD, it's far more noticeable than other games. Having discussed this at length on the forums, my experience is that only about 20% of winning games in Black Ops comes from skill, with the other 80% being purely down to how good or bad your connection is. I know what I will hear back from Black Ops fans is "it's much better nowadays"...perhaps it is, but after investing 50ish hours in the MP, and finding it lacking, I have zero interest in going back to find out. The rage I experienced from getting nothing but hit markers for a clip and a half before killing someone make any anger I ever felt in MW2 pale by comparison.
I freely admit that MW2 has a lot of faults, however having played both games for a significant period of time, I don't see how Black Ops has improved on MW2's faults in any major way - aside from theater mode of course, and that wasn't a fault in MW2 as much as an omission. The whole needing to get killstreaks with gun only is a nice idea in theory however it has done little to discourage camping - if you like to play like a little bitch in MW2, chances are you'll play like a little bitch in Black Ops.
And the clincher of course, the reason we all play games, and the reason why MW2 is a clear winner for me: MW2 is just more fun, dammit!