Can't tell for sure...or more precisely, it depends on what you're looking for.
If you're bothered in jumping on the SLI bandwagon, you're only choice is the 680i [ you can have SLI on i975X & RD600 with "modded drivers", but only possible with the GeForce 7xxx series, 8xxx series isn't supported ]
If you're going for CrossFire, you've got two "pure" choices...the RD600 & the i975X.
The i975X seems to perform better ( clock per clock ) [ overall ] than RD600.
On a single VGA scenario, we got:
in 2D ( Applications, etc ) : i975X > i965P => 680i > RD600
in 3D : i975X => RD600 > i965P > 680i
[ average of various tests ]
Things may change though, especially with the new motherboards coming and new BIOS revisions.
[ performance tweaking-wise on the DFI ICFX3200 { RD600 Chipset } & the DFI 680i motherboards feature a lot of tweaking options in BIOS, and can clock our RAM better, such "equalizing" the performance ]