Both brands are fine, they normally use stock reference cards with little modifications to keep pricing down, also most of the time the GPU is the part you replace more often then not so warranties shouldn't be the driving force behind buying a specific brand, especially when the pricing is R300 more in some circumstances. If you can get the 6870 it is a pretty amazing card, I picked one up for R2600 last month and wow has it absolutely slaughtered my 4870 it replaced, I did a lot of research into either getting the 6850 or the 6870 before settling on the 6870.
Also remember SLI isn't going to give you a 100% scaling on your system yet, so you might only get 150% over the norm, but I have heard 460s in SLI outperform a single 480, the 5770 is a decent card but it doesn't compete with the 6850.