mmininginc - I have the ASUS R9 280x DC2 and 100% happy with it.
Like claygraffix I get 750kH/s with it. (undervoltaged + increased the engine clock with 80MHz)
But what I really like about this card is its superb and silent cooling system.

claygraffix - I have 4x ASUS R9280X-DC2-3GD5 3GB GDDR5 DVI-I DVI
All at 750kh/s on stock settings, 69-71 deg.  Going to buy 4 more soon.

Prelektro - i would go for asus 280x. toxic is hard to cool and getting really hot and noisy

I agree with these guys.

Running 6 x Asus R9 280x with two more arriving any day now.

745khs out of the box on stock. Voltage is unlocked so you Undervolt to save power and heat which lowers fan noise as well.

Don't recommend the Sapphire Radeon R9 280X Dual-X as they throttle at 72c or something silly.

Go with the ASUS, you wont regret it. I haven't