Barring any personal experience with these blowers, I'd say get the one with the largest CFMs -- the MassCool. However, you have to realize that fan manufacturers are sometimes unscrupulous in their specifications. Both of these supposedly generate dBA noise-levels in a low and tolerable range, if you can trust the specs. But I've heard that these blowers can be noisy, nevertheless.
Personally, I would look more carefully at the Arctic Cooling Silencer model which fits your 6800 card. I personally know at least one person who ordered the "proper" model of the Silencer, only to find that it still didn't fit his card. I was more lucky in finding a Silencer to cool an nVidia Ti4600 AGP. It "just" fit perfectly.
One reason I've never chosen the blowers is the loss of a PCI slot, but either the Silencer or a Zalman heatpipe cooler will leave you bereft of a slot, anyway.
I would take a closer look at Zalman ZM80D or similar models which fit your AGP card, and consider using the OP-1 option fan, which, like children, can only be seen and not heard even at 2,900 rpm. ThermalTake also has a new model called the "Schooner", but it doesn't offer a fan option in its passive cooling design, and assumes airflow outside your case is sufficient to remove heat from the external fins.
Great things have been said in recent days here at AnandTech forums about the "Schooner". Personally, I can say that the Zalman heatpipe coolers have cooled down my AGP cards from around 120F to the mid-90's range -- with the help of the OP-1.