The main tube size(30mm) will not do anything to the brightness or clarity, it is really just more to due with strength gain over the 1" tube. In most cases the 30mm gives more adjustments than the 1" tube as well. The brightness and clarity has more to due with the glass and the muliticoating they use on the glass. The 50mm will gain less than 2% over the 42mm in light gathering abilities, comparing apples to apples(same model, glass and coatings used), so yes it should be brighter but is it going to make a difference with quality optics and usable under legal light probably not.

I think SJ was saying the better the optics(glass & coating) the less the useful gain in the objective size for brightness.

It is really a matter of preference and what you need the combo for, if a higher mount will force your cheek off the stock then it is better to go smaller, if you need to pack it on a horse or carry the rifle the small/compact issues will take effect...if none of this applies to you then you can go with either.

I know very little about Kahles but there lifetime warranty is (or at least was) not transferable like many other manufacturer's. Which is the only dig I have really heard but it does make it less appealing to those forking out top cash for a scope.