The HIS usually has some problems with heating but since your getting the ICEQ edition I think you should be fine. Just looks at some of the reviews for each GPU. As for clock speed I think the HIS has a higher clock speed because its turbo and the other is not.
So look at some reviews and they should give you about the same performance.
If it was me then i would pick the sapphire HD 7850 only because Sapphire is a more reliable cooler than the HIS.