Both are 20mm but I've never had trouble with light gathering on the 1" version....
I know the illuminated version with the 30mm tube is listed as a 1.5-5x20mm, but I don't see how they can use the same 20mm objective lens in the 1" scope and 30mm scope. If they did, the tube in the 30mm scope would need to be about 10mm thick (i.e. 50% of the diameter of the lens). I figure that is why many/most other 30mm scopes have a 24mm objective lens.
I very well could be wrong, but I have a suspicion that the 30mm scope has a larger objective lens than 20mm....
