Over here in the uk the most popular slow speed 6inch grinder seems to be the Creusen HP7500TS Powerline, now it ain't cheap but it's a good reliable well built grinder (should last a lifetime if shown a little respect!), not sure if it's available over in the US ?
I too would side with the CBN wheels they are great, I have a 6inch 100grit one they cut fast and cool and stay the same size, again they are not cheap but if treat with respect they should last a VERY loooonnngg time, worth every penny !!!