For a second I was surprised by the positively dominating lead of MW2; that is, until I realized there were only 5 votes xP
But yeah, I prefer MW2 because there's a lot more game there. There's more weapons, a lot more perks (when you take into account Pro versions) more killstreaks, and it all adds up to be my favorite thing of all,a better way to customize your particular classes, really make them yours.
In COD4, more than half the people had stopping power, martyrdom, and an m16. And I don't even mind them in that I thought they were overpowered, I just didn't like the lack of variety.
Also, titles are fun.
EDIT: I think it's funny that (now) the votes are evenly matched, but every single post supports MW2, there's none defending the first one.