Steve - when I bought my 3M, I visited Axminster to make sure I was getting the right thing. I discovered an unseen difference in the two helmets, which is that the webbing skull cradles are completely different. I bought the 306 for the sole reason that the 106 did not feel that it would stay securely on my head, whereas the 306 is deeper and far more adjustable.
My advice, for what it's worth, given that you will be shelling out soooo much money, is to do the same if you can, and visit somewhere that stocks both helmets so you can try them on. They fit and feel completely different.