The 30mm has more advantages all round, these are better light transmission lens quality and coatings being equal. more windage and elevation.It is easier to align. the larger diameter lens during manufacture. contoured lens are better if they have a reduced angle due to a larger diameter. a thin walled tube of a larger diameter is stronger than a thin walled tube of a smaller diameter. For some reasons the Americans seem to think a 1" is still better this is usually due to ignorant gun writers in the magazines and a reluctance to accept that anything from outside the states can be better,superior or invented any where other than the states