I have had Asus products and Sapphire products and have had one RMA with Sapphire a totally painless process. Sapphire happens to be the biggest producer of ATI cards. In this case I would choose Sapphire.
Well, since I'm assuming you're talking about reference models they are exactly the same except for box art and bundle. The GTX 470 may force a price reduction for the 5850, but only if they are released in significant quantities. I guess we'll find out in a few days.