Although the single player in MW2 was good I preferred MW.
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In the first one it was great when you were the American chap and you snuffed it in slow mo. They obviously realised this and repeated it about 6 times in the second one which kinda removes all emotion from it.
I didn't like the imaginary All American 'Task Force' they set up. I preferred the first one when you were playing as the actual SAS and Military, seemed more real.
I thought the invading America was frankly stupid.
And I hated that level in Cuba in the shanty town.
I didn't think the weapons were as good. For some reason I just didn't feel like I got enough feedback, not as satisfying to fire.
Oh and well we're on it, (a bit picky I know) but at then end when a fully trained SAS bloke is getting the **** kicked out of him by some old fat general I thought it was a little silly.
The snow level was rather excellent though, possibly on par with the Sniper Level in MW1 which is awesome
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All in all I'd say start with MW1, if you really like it, (which you will as it's a classic) buy MW2. You'll enjoy the SP and MP on MW1 more but you wont be upset by the SP on MW2.
As it stands though the MP on MW2 is fairly awful, as well as the whole cheats / dedicated server issues, for me it is too much like a console game, gamer tags and too much reliance on drops / power ups, (I'd say the maps were about on par though).
I still play MW1 for the MP, (which is still busy by the way, you'll have no problem finding a server) and it is still one of the best around.