If you like the Ultimate Slam, a VX1 or VX2 will be a great fit... about the same with duplex rather than bdc clutter. I have 3-9x40 on all my ML's. Just works real well all around for me. I also don't like much clutter, all my scopes have duplex except 1.

There would be no reason to make a 40mm or a 50mm if they didn't let in more light. All scopes would scope tube size. At least if you use higher magnifications. If we all used a fixed 2X scope nobody would need a 40mm scope.

Depending on the quality of the scope and its coatings, there can be very little difference between a 42mm scope and a 50mm scope, as far as how well you can see at dusk. The human eye is capable of just so much and with the high end glass and coatings on some scopes today along with the human eye capability, there's no measurable difference, say between a 42mm and a 50mm.
I have two Leupold VX6 scopes, one a 44mm and the other a 50mm and there is no visible difference in low light, light transmission.
Larger scopes, in most cases require higher rings, which may or may not help the shooter.