the HIS probably does have better cooling, however i've heard weird things about their cards having a higher idle clock for some reason. both are kind of overkill for the 5670, which does fine with smaller single slot coolers. i have the sapphire and it never goes past 55c at load.
also, if you were planning on getting the 1gb for either, i just thought i'd mention that you won't see any increase in performance over the 512 mb version at the resolution you're probably running. and if you were to be in a situation where it did offer more performance, it'd only be at unbearably low framerates. i dunno if you were gonna get the 1gb, but just in case.
that being said, i don't think the 5670 is really necessary for you. it has a sorta specific niche for people that want to get the most graphics bang out of their cheap psu's. for the same price on newegg you can get a 4850 which is leagues above the 5670 in performance. it consumes more power, but you're psu should handle it just fine.