I slightly prefer KZ2, mainly because it feels different - literally due to the controls and movement weight - while MW2 feels like the latest version of something I've played since the Doom days. MW2 is probably better overall for MP, but I'm just a bit bored of unrealistic controls and movement now, I seem to be seeking something that feels more real.
I do miss coop though. I tend to play a heck of a lot more coop than MP, so that was a big loss for me, and I still don't understand why they couldn't have online coop - I do understand why they don't have offline coop.
Try to be reasonable... its easier than you think...