I was reading on another somewhat gun related internet forum... where a person asked this very same question. Naturally, he got all sorts of misinformation, followed by arguing as to who was right and who was wrong... then the thread pretty much turned to crap.
So... here is what I can tell you for sure. The difference between 1" and 30mm tubs is: 4.6mm, any more than that is pretty much a guess.
So, let's look at some of the common assertions and beliefs about the 1.00" vs. 30mm battle... which, I believe is discussed largely because the 9mm vs. 40SW vs. 45ACP thing is just so old now.
The adantage to 30mm is that it offers more adjustment for elevation: Well, it can, sure... but there is another tube inside of the 1" or 30mm external tube, as well as the associated adjustment assembly, and in a lot of cases, scope manufacturers use the same "guts" in a 30mm tube as they do in the 1" tubes. So, just because a scope has a 30mm tube, it does not mean it has more adjustment, but the only way one knows what the adjustment range of a particular scope is, is to get that spec for that scope.
30mm tubes most likely caught on because they were "European" and associated with high quality scopes, or maybe the "more is better" mindset... American scope companies had to ofer 30mm tubes to compete -- but the smart buyer will check to make sure that the only feature of that 30mm tube scope, is not the 30mm tube.
30mm Tubes are stronger: Well, again... they could be. But just having a larger outside diameter does not assure that a tube is stronger than a smaller diameter tube... Given the number of scope tubes that I have seen break, this one really does not matter that much to me one way or the other.
30mm Tubes are "brighter": This one needs to be broke down into a couple of catagories...
1. A 30mm tubes will/won't pass more light than a 1" tube: This one is silly, but people argue it... of course a 30mm tube will "pass more light" -- to put this in the best "Are you smarter than a 5th grader, home science project" just ask yourself this:
Imagine you have a little solar panel, one of them things you shine a light on and it makes 'lectricity, and you masked it off so that only a 1" hole was uncovered and put it under a light and recorded the voltage produced.
Now, you do the same thing, but with a 30mm hole... do you think the voltage would be higher? Course it would, there is more surface on the panel exposed to the same light -- therefore, there is more "light" getting through the larger hole.
Of course that is a silly example, and not at all the type of light transmission that we deal with in a rifle scope... so, lets look at the real answer.
Yes, a 30mm tube can be brighter than the same scope with a 1" tube... can be, most likely is, if it is an expensive scope because they will use premium optics, coatings and engineering -- optics, glass and coatings aside, the increased room in the 30mm tube allows for the use of larger Field Lenses, Erector Lenses and other optics in the tube. Larger optics can receive and pass more light. But then you start to give up some of the increased adjustment room, doh!
If the manufacturer does not use larger lenses inside the tube, then there will be no difference between a 1" and 30mm tube as far as "brightness" goes. If the manufacturer (properly) uses larger internal optics, the scope can be brighter... will you be able to see the difference? That depends on the user, the conditions and the scope.
30mm Tubes are 4.6mm bigger than 1" Tubes: Yes, that's right! That is one of the fo'sures... also, that the 30mm scope will need 30mm rings! Truth is, a great number of scopes with 30mm tubes are just that... a scope with a 30mm tube, but folks buy them and it sounds cool when you post on the internet that you have one... Not saying that 30mm tubes are bad, as I pointed out, they may in fact offer a superior package inside -- or they may not... caveat emptor.