Sorry for the threadjack
If you want an honest simple answer, buy what you think looks best, and what you're most comfortable handling. The differences in the way they perform out of the box aren't even worth mentioning.
Personally, if I were the sniper and had to choose between an M4 and an M16, I'd go for the carbine every time. I feel that the shorter overall length is of much more benefit than any potential accuracy gains. It is about 50 times easier to crawl through and over brush with a 7" barrel (the CQD I use currently) than a full length 20" barreled assault rifle like an M16 (G36 in my case), especially since I'm a smaller guy.