Both cards are very close in performance with the 7850 being perhaps a bit stronger.
You will not be able to tell the difference while playing games.
For gaming, buy the strongest graphics card you feel comfortable paying for.
Use this heirarchy chart as a guide:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Do not worry about 1/2gb. That makes little difference in performance for the most part.
Be on the lookout for some AMD price cuts; they want to compete with the newest Nvidia cards.