Ohh well there's always pros and cons about using a dual 6970 setup:
two 6970s would cost the same as a 6990 so that is pretty much the same thing
Pro: 6970s will run cooler
Pro: you'll get the full 880MHz and not the underclocked 830MHz of the 6990
Pro: you'll have higher overclocking headroom
Con: they'll take up 4 PCIe slots instead of 2
Con: you'll need 4 6+2 pin connectors instead of 2
Con: you'll be limited if you wish to add further capacity in the future
I hope that helped you out a bit, I'm not much of an expert myself but those are the points I could make for different setups. I need the most rawpower I can get, and I'll get about 11 TFLOPs with dual 6990s in quadfire mode. In my case, it's justified to go with 6990s instead of 4x6970s because then I'd need 8 6+2 pin connectors, which would mean a much more expensive PSU, I'd also need to get a Full tower case, and I'd have to go with an E-ATX mobo. So the cost would really add up. Besides, I don't know how much cooler 4 6970 stacked up on top of each other would run compared to two 6990s with a slot in between and good airflow.