Depends on the price:http://www.anandtech.com/show/5625/amd-radeon-hd-7870-ghz-edition-radeon-hd-7850-review-rounding-out-southern-islands/11
I use a standard price algorithm for price/performance, percentage difference in fps vs price.
According to the link provided, it's at *least* 5fps better (6870 = 55, 7850 = 60fps) which is 10% better.
So, at this stage, due to the less than 10% price difference (here in NZ, 6870 = 360, 7850 = 380) it becomes better value for money.
Get the 7850.
As for brand, my Sapphire 6870 = awesomeness. Quick, and very quiet, so highly reccomended. In particular, look for the Vapour-X branded ones, they're consistantly rated amongst the best factory coolers available.
A cool card will last longer, and (of course you are going to..) better for overclocking in the future, thus worth the small difference over an entry level device that has the stock (read:noisy and hot) cooler!