PCI-Express v2.0 cannot be found on the 650i. Although it's not necessary, and probably won't be for some time I recall that v2.0 has power management features that v1.1 does not.
Other than that really nothing as I'm pretty sure they use the same chipset. If you're going to SLI make sure that either board comes with PCI-Express x16 in full-bandwidth -- x16/x16 and not x8/x8 for each bus -- when SLI is enabled.