There are few American scopes that use 30mm glass in a 30mm tube. Most just use 1" glass and offer you more range of adjustment. Unless you are one to make in-the-field scope adjustments a 30mm tube isn't going to gain you anything.

If you want true light transmitting performance from a 30mm scope then you are likely going to have to get a European 30mm scope. Such as Zeiss (not conquest), Swarovski, Kahles, etc...

The one thing that a 30mm tube has over a 1" tube is strength. No matter what glass is inside a 30mm tube will be considerably stronger than a 1" tube of the same construction.