This is kind of dated now, but my understanding is for 100-300 yard ,6ppc tends to shoot better. For 300-600 yard , 6BR shoots better. I very well could be so so wrong though.
There is a compelling reason 6 BR's are rarely seen at Group matches. I have a 6 BR HBR rifle I've used off and on over the past 3 years. I have won 4 matches with it. The matches I have won with it was when the wind was switchy and strong and those by more then one point and or x es. When it was new, I felt it shot tighter than the 30 BR I usually use but some days it really shot poorly = out of tune I think. Once in awhile one sees one that really shoots well. I'd say that is a rarity though. The "goodie" in the barrels doesn't seem to last a long time. It really gets down to Barrels I believe. A good barrel chambered in 6BR will perform.