Here are the two views;
Offline:
Cod4's campaign is so much better compared to CoD6/MW2.
MW2's campaign was fun, but there was no AC130 "wow" mission, and there wasn't the in-depth stealth sniper mission, granted there was a short little opening with on in it.
Also, CoD4 has cheats and Arcade mode, which MW2 did not repeat (because that would be fun).
The only thing MW2 has to offer over 4 is spec ops, which are fun, though some require 2 people to play.
Online.
MW2. Period.
Not trying to be a fanboy, but its basically CoD4 with more guns, perks, and all around improved.