If the performance is close then i get the 9800gx2. There is no reason to spend double the cash for a meager improvement. That's just the reasoning unbiased.
Truth is: the $500+ price for the 4870x2 is justified. I dont know what benchmark you been reading but you need to do more research.
Why would ATI mark their card so expensive if performance is close? Didn't you noticed Nvidia dropped their price for the top model cards? It is due to performance vs price ratio.
If the 9800gx2 performs like the 4870x2-it would be $500+ too