The GTX 680mx is a bit faster than the 5870. A tier or 2 higher in the GPU chart (GPU chart only has GTX 680m).
But the Mac Pro is a much newer model (not sure how that affects Mac OS) and uses MXM GPU, so even if the older hardware can be upgraded to the newer Mac OS, the newer iMac saves you a lot on electricity with slightly better performance. The GTX 680mx, because it is an MXM card, it also has a much higher resell value (but much smaller market) than the PCIe 5870.