Got one 280x Sapphire vapor-x and Asus 280x DCII TOP. I would say the hashrates are pretty much the same (no custom bios just 1050/1080 and 1500)
But when it comes to heat and noise... the difference is day and night
Asus blows sapphire out of the water so bad it aint funny. ++85c core and +110c VRM's on the vapor-x! Fan noise is also terrible. Mind you i kept them on my normal Fractal R3 case with 5 fans but still. I can run the Asus at 50% fan while staying at 75c full load (VRM max 75). I would get the Asus for rigs without cases and pretty much anywhere if it were in stock. Sapphire Vapor-X if its the only thing you can get your hands on with mandatory undervolting and good overall cooling.