i prefer mw2, cod 4's quite good as well apparently, i never played it that much. I like to play for the kill streaks tbh and thats why i played mw2 more. The modders have apparently got to cod4 as well.
mw2 is by far the best most if not all people who dislike mw2 either are obsessive with hardcore and dislike something about mw2[ what idk hc is hc in anygame for me] or they just plain are terrible or cry about the kids who are good enough to get killstreaks, as long as ur decent you will love mw2 above all else.
COD4 mate, MW2 is 98% dead on PC, plus the online matchmaking is broken and I doubt it's gonna be fixed. If this were 2009-2010, I would have said MW2. However the campaign isn't that bad, nor is spec ops. But for online, go with COD4.
Cod 4 Gameplay itself is a ton better. A lot more balanced, and just enjoyable. However it does seem like there are a ton of people with hacks/infections which makes it very frustrating to play.
I personally love MW2. Never tried COD4 yet but I heard it is also very good. The only thing is the amount of hackers,etc. now. I'd go for MW2, but I guess that's my opinion.
I love both so much but I enjoyed MW2 more because it wasn't as repetitive. There were a lot more guns and maps and other features which made it a bit more enjoyable.