Because I use MM Cobs for now I am using the 6mm and seem to good with them. I did order some 6mm balsa inserts but seemed to reduce the taste but I have 50 of them so will keep tailing them,
For 9mm is hearing extremes where some people love them and feel
It’s helps with moisture and improves taste, while others feel it hurts taste.
I will try all as I acquire pipes and will probably just end up with them all.