The difference is memory and video. You really won't see much difference with the memory, but the 7850 is a batter card. I would take 8GB of memory and the 7850.http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
16gb of memory won't make much of a difference at all if any. The average user only uses about 5gb. Yes, The 7850 is definately a better card and that is what will matter in games.
Enjoy your pc!