What do you plan on doing with this rig?
The answer very well may be neither. Your current Crosshair III Forumla MB will do CrossFire with both the 6970 GPUs running at PCIe x8 (if I recall correctly). The differences between dual x8 and dual x16 (with a 890FX MB) is minimal. Also, four cores running at 3.91GHz is plenty for gaming. IHMO go with what you have. Upgrade memory or to an SSD if you have money to spend now. Upgrade MB and / or CPU down the road....
The crosshair 3 has dual x16 lanes and I plan on gaming mainly