what really comes down to is price tbh, extra heat and power draw isnt gonna kill your card or blow your power bill, I was going to buy the GTX 470 but at the time in my country it was priced more near the 5870 than 5850 so I went for the 5850 and I dont regret it,
the 5850 is a great card, most of my games are 60+ and the ones that arent like metro 2033 is very much playable at 30-40 frames all very high but DOF disabled all that on 1920x1080 res, also dont worry about things like heavily tessilated heaven test and all that, it will be a long time before games actually use that much DX11 technology in their games at least till the next generation of consoles which is predicted at 2012.
so take my advice both cards are great and really they both perform similarly that your only consideration should be about price, btw SLI on the GTX 470 is much better than 5850 crossfire but I am not a big fan of either SLI or CF, I prefer to buy new tech every two years than be stuck with the same tech for more than 4 years