I like the adjustment option... I do shoot out to 1000yds on occasion. Also, 30mm is better for light transmission.
This has been stated previously, but it bears repeating: tube diameter has no bearing on light transmission (i.e. the brightness of the image, low light performance, etc...).
Light transmission is determined by the design of the optic system, the quality of the glass (composition, clarity, polishing...), and the effectiveness of the lens coatings. Similarly, light gathering/collection is a direct function of the diameter of the objective.
Tube diameter provides more room for adjustment (this is why top-end long range scopes use a 34mm or 35mm tube). Optically speaking, the only improvement a larger tube can offer over a smaller tube is if the larger tube actually has larger lenses installed, which will allow for a little more up-and-down or side-to-side head movement...but it will NOT improve the amount of light transmitted nor result in a larger exit pupil.
Basically it's like this: every optic throws a disc of light known as the exit pupil. The size of that exit pupil is the objective diameter (in millimeters) divided by the level of magnification you are at: 50mm objective at 10x = 5mm disc of light or exit pupil. If the size of the exit pupil/disc of light is equal to or larger than the current diameter of your pupil (typically 5mm), you are experiencing maximum exit pupil. Any exit pupil over the diameter of actual pupil will not result in a brighter image, but will provide more eye relief.
Now here is where light transmission and the quality of the optics system, glass and coatings are relevant: a higher quality scope will throw a clearer, brighter disc of light than a lower quality scope. Optimally, you want a rifle scope with a light transmission of 95% or better.
The "myth" of 30mm tubes collecting/gathering/transmitting more light that 1" tubes seems to have originated from the fact that back in the day, the better European scopes used 30mm tubes while North American scopes used 1 inch tubes. As the European scopes were, at that time, built to a higher standard, with better glass and technology, they were unquestionably brighter and clearer. Some figured this was at least partially due to the fact that Euro-scopes had a larger tube.
Hopefully that clarifies things.
Best bang for buck scope available at the moment?
Swarovski Z3 4-12x50 with ballistic turret: regularly $1,599, on for $1,155.