Mw3 is a lot more different than Mw2 then you think. MW2 would be a lot more skilled based than MW3. Although there are many perks that usually base how you are going to play in MW2, in MW3 its more class based. Say you make a rushing class (MW3), extreme conditioning, quick draw, and steady aim with a UMP with lets say range, and you pick up a M4. You wont necessarily do well with the picked up gun or with a slower paced safe style, you'll have to rush. Where as in MW2 you can pick up another weapon easily and adapt to the style of the weapon.

As far as MW3 weapons can go they can become pretty accurate, depends on your class load out. One of my favorite the MK14 is amazingly accurate, with red dot sight and kick that shit can go HAM. I'd say the MW3 weapons depend all on your style. They can more accurate than MW2 depending on your proficiency. Which is the one reason I dislike the proficiency system, too much change to the gun and less skill oriented. Players that suck at accuracy shouldn't be able to put kick on a weapon with high recoil. That is the nature of the weapon it shouldn't be changed.