I have used both Asus and MSI ATI based video cards and have had great success with each. As Lmeow suggested, comparing the warranties is a good idea, as is comparing the cooling system of each vendor's card.
If you want a quiet card, look at product reviews of each for comparison. Newegg has one of the better review systems on the market and a quick scan of current reviews can let you know if a particular vendor's solution has problems.
Both vendors make good stuff. If still a toss up, I would go Asus as well.
Enjoy your new GPU!