Re: Which would be brighter?
The brightness--all else being equal, is determined by the size of the exit pupil. The exit pupil is the little beam of light that exits the scope at the eyepiece (occular) which you hold you eye up to. You can see it on a piece of paper if you hold the scope toward the light and hold a piece of paper about 2 to 3 inches from the occular lens.
The size of the exit pupil is determined by the diameter in millimeters of the objective lense (the big lens at the front of the scope) divided by the power that the scope is magnifying at.
For the fixed 6 power scope with a 42mm objective, the exit pupil is 7, which is considered to be perfect.
A 4 power scope with a 32mm objective would actually have an 8mm exit pupil--actually more than big enough.
So with the variables you mention, it depends on what power they are set on. As you increase the power on the 18X scope, you'll have an exit pupil equal to 42/18, or 2.3mm. That's pretty small, and such a power level would likely only be useful in the well lit, bright day--when your pupil is constricted.
On the other hand, the 56mm scope at 15X would have an exit pupil of 3.7mm. Still on the small side, but certainly brighter than the 18X scope with 42mm objective.
It almost sounds like you're trying to decide between an IOR scope and a Nightforce NXS. [img]/ubbthreads/images/graemlins/wink.gif[/img]
I sold some extra stuff I had laying around and went with the Nightforce 3.5-15 x 50, knowing that for all practical applications the 50mm objective would be plenty.
If you get the Nightforce, be sure the get the NP-R2 reticle, which is divided in MOA. If you're on the American measurement system, the MOA reticle rules. See my post elsewhere in these pages as to why...
The tube diameter has nothing to do with brightness. I believe the IOR's 35mm tube was a necessity because of the six times zoom number (3 to 18). Probably since the erector tube is longer it needed more room to get enough elevation adjustment to be considered a long range tactical scope.
The NXS is a three times zoom. Low power is 3.5 (estimated). Shorter erector, more elevation "wiggle room" inside that 30mm tube.
For what it's worth, the Navy Seals chose the 3.5-15 x 50 NXS over the 56mm version.