Do you have the power necessary to run 2x 6850s? 2x 6850s are more powerful than a single 6970 for sure, but that's it in terms of upgrading. I'd suggest the 2 6850s if you're planning on building/buying another computer in the next couple of years, otherwise I'd get the 6970 and add another when it gets time.
What motherboard and power supply do you have?