But if you have plenty of money to spend and X number of pci slots to fill, why wouldn't you use 6990s? If someone is actually building a mining operation that is around ~10,000 cards big, that means they probably have some substantial financial backing. They are probably going to need space to house and cool at least 2500 rigs (assuming 4 cards per mobo). So in a situation like that, your 'hashing power per square foot' becomes a bigger and bigger factor.

Your overhead costs would be much higher to house and AC the same amount of hash power using 6970s as opposed to 6990s. So even though the initial cost per mhash is higher on a 6990 compared to dual 6970s, the 6990s would be much more efficient to mine with in the long run. Overhead cost per mhash is often more important to consider than purchase cost per mhash, you only buy the card once but you pay money to run it every second after that.

I would definitely prefer the 6970s, but that is just based off of my current situation.