This will be an age old arguement but IMHO when talking about a 3x9 or smaller scope, the 40 is as big as you will need. The 50 does present mounting "concerns" (wont call them a problem as it's simply a matter of taller rings) but there is a point of diminishing returns. I DO however think there is a time and place for a 50mm bell and that is when dealing with scopes of 12 or larger power. They need the extra width in the pupil that a 50 gives. A 9x does not. When looking at ANY type scope to be considered for dark situations, simply divide the objective lense (in MMs) by the power and the number you end up with will be the metric size of the scopes pupil (just like your eye). Ideally you want something in the 3.5-5mm range with 4-4.5 being about as good as you need for a hunting scope.

THIS is why European scopes of high magnification have 30mm tubes and 56mm bells on them. They need the moonlight gathered (they hunt alot at night) when the scope is cranked on up there. That's not a consideration for a 9x being used stateside (at least not to the legal hunter, hehehe).

If you are just SWORN too the most without high rings, then look to Nikon or Zeiss Conquest and check out their 44mm line. Thats about as big a scope as you will put in medium rings!
RA