XFX - Pro: Double lifetime warranty if you ever want to sell it. Also they are a pretty decent company overall.
- Con: No voltage tweaking to my knowledge so overclocking overall, then again 6870 doesn't OC much. They don't have custom 6870 coolers so they'll run hot-ish.
Sapphire - Pro: Good well respected company, custom coolers. Trixx or something like that for Sapphire card OCing, and also pretty decent overall company.
Con - Meh when it comes to support. They're alright, not as good as XFX to my knowledge.
Asus - Pro: Assured voltage tweaking, OC's great. Custom cooler is the best of best. (Not sure how it compares to the Sapphire non ref) The DirectCu is my choice but a biased opinion. Runs cool.
Con - Warranty isn't as good as XFX, Asus seems to be alright at customer service, but again. Not as good as XFX.