we've had experience with both wheel sizes.
we used the 8" for Rack 'n' Roll, and the 6" in Overdrive.
the 8" were great, we had no real issues with them. The 6", we had problems with the plates deforming and digging into the carpet, killing our turn ability completely.
as for your ramp question;
the 8" wheels have no problem with the ramp at all. we tested this extensively today. the robot we were using to test, even realigned itself when we were not perfectly on target
the two different wheel sizes handle differently i'd say (i drove for Overdrive, and had practice with the 8" mech robot). But that's probably more to do with the fact that they're designed differently, and so they function slightly different.
hope that helps
