Using SMG cause more deaths due to agressive gameplay. When I'm playing with SMG I usually get 25-40 kills and counter much more enemies. While using AR I play more defensivly/semi-camp. I get 20-30 kills but die rarely.
So overall scores are much lower with ARs than with SMGs...at least this is my experience.
Yeah, this is not even close to being sceintific. For one thing, the sample size is still incredibly small, and as you pointed out a (decent) player will change their gaming style and become a bit more defensive when using an AR or LMG vs. an SMG or shotgun. But even with so few people involved so far, you can see a few patterns developing:
For one, Treyarch apparently has done a great job in balancing the weapons. The only primary (not counting the assault shield) which nobody has mentioned is the KSG, and there's a tremendous amount of variety in the weapons each person has named.
Additionally, you can see that when taking only k/d into consideration there isn't really a disadvantage to using any class of weapon, obviously varying based on individual skill level. The sniper rifles tend toward the bottom of the list, but far fewer people use them vs. the other classes...for those who did mention snipers, they generally ranked very high on their lists.
And of course you can start to get an idea of which weapons might be better (or simply easier to use) within each class. The AN-94 and FAL OSW are running away with things among the ARs at this point, while the PDW-57 is well ahead of the other SMGs.
Interesting stuff at any rate, and hopefully it will become even more interesting as more and more people respond.