30mm tubes offer more windage and elevation range, but not always depending on how the scope is designed. The potential is there, though. It also is more structually rigid than a 1" tube.

Light transmission is a wash. A properly designed 1'' scope can transmit virtually the same amount of light as a 30mm scope if all else is equal, meaning the same objective lens diameter, same glass, same coatings, etc... The really bright European scopes are generally the ones with big 56mm objectives and more expensive glass. All lenses are not equal. More effort and better materials go into the really good lenses so they (the lenses) can correct all the aberrations (chromatic, spherical, coma, etc...) that effect image quality. The more expensive lenses that are used in the higher priced scopes (which most European scopes are) do a better job at it than those used in medium and lower prices scopes.