You'll get more Mhps out of a 5870, so if you find a good deal on one, that might be better. I've had better luck finding deals on 5850s though. A 5850 will do 350-400 Mhps, and a 5870 will do 400-450 Mhps. But I think it's easier to get a 5850 to 400 than it is to get a 5870 to 450 (saying that just based on my general impression from reading around in the forum). Another consideration is efficiency if you care about that. 5850s are known for their efficiency, only pulling about 150 watts. But if you are just getting one card, that isn't much of a factor.