Like all of the opinions I'd go with the 5850. Remember though all the prices are dropping. The 5850 you really want to look at are the reference designs or the Asus DirectCU or Mayyybeee the MSI twin frozr.
The reason for those specific 5850s is because the Asus DirectCu 5850 and reference design 5850's have unlocked voltages. The reason unlocked voltage tweaking is so useful is most people who have a 5850 that can tweak the voltages can OC the 5850 to 1000 core clock which at that point it scores 18125 on 3DMark Vantage (I know this because I've tested it). The reason I wouldn't choose the Twin Frozr is because although it is somewhat cheap, it actually runs hotter than the reference design.
Asus DirectCU 5850 TOP review: http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/35922-asus-radeon-hd-5850-1gb-directcu-top-review-15.html
There is the version without the "Top" which is a lot cheaper, the only difference is the TOP is factory OC'd a bit, but still either the model with TOP or not the DirectCU is a best as it is very cool.