When I built my first 6PPC (in 1979) the 6BR was still under developement. I got reamers for both BR cartridges (22 and 6mm) soon after and built a few for other shooters. I stuck with the 6PPC though. At the time, Remington BR brass was 308 brass with a small primer pocket and it took considerable effort to form it to 6BR. Lee Six's daughter, Cindy, sold formed brass and it was worth it to pay her rather than to do it yourself.
If Remington had produced 6 BR brass at that time, the PPC might not have been quite so dominant.
Presently, I shoot a 6BR at short range out of sheer obstinance. It is competitively accurate and, with the excellent Lapua brass available today, it is a viable round for short range use. I am usually the only one shooting a 6BR however and if pressed, I will always recommend a 6PPC. GD