The difference is .1811 inches. Seriously though, I believe they use 30mm tubes when wider windage/elevation adjustments are needed. Some claim they have better light transmission also but I doubt that there is a noticable difference there. So I would say if you need a large long range scope with lots of adjustment capability then buy the 30mm. If you need a "normal" hunting scope, get one with a 1" tube and save some money. The 30mm scopes are not better quality - just a different purpose.
