As said it would come down to which was the better deal. Things like, does one hav ea better cooler. Is there a better bundle with one, such as is there a game voucher or better selection of cable included etc. How long and how good is the warranty.
I would not say one was a better make than the other and so if they were both more or less the same then either as sugested flip a coin or go with which ever one you loke the looks of best.
I have used both brands with equal success. As others have indicated, look for the deal that provides the best features for the cost and you really can't go wrong with either brand. Be sure to compare 5870s to 5870s and 5850s to 5850s.