the GPU itself is made by ATI, saphhire and visiontech make the PCB, the cooler, and package it and stuff. The memory is made by a 3rd party. The 4870 chip is the same for all makers and the 4890 is the same as well. They may have different specifications, like some companies will overclock and things like that, but its generally the same. If the sapphire has the better cooler that'll mean that the card will run cooler, but it won't mean that its faster. But obviously you'd want your card as cool as possible, but just because the heatsink looks cooler it doesn't mean it cools better.. you never know... look for product reviews for the visiontech to see how good their heatsink is. They both look like good deals to me.. and i'd lean towards the 4890.
hope that made sense, because it did in my head