Right know AFAIK, all the vendors are producing just "ATI reference" designs. If that still holds true, I'd be hard pressed to identify a physical difference in the cards. I must say however that since I'm not shopping till mid winter, I am not looking at anything very closely atm, just regurgitating what I have read in the trades.
All things being equal however, I'd match the vid card vendor to the MoBo vendor (i.e. Asus MoBo , gets Asus GFX card) so at least when you call TS for one of them, they can't blame the other vendor