The 40mm lens is a tough call.
It "may" work on the Sixtub, but it may not. If I had a 40mm lens, I would first check it on the Sixtub, as it is shallower and thus easier to reach the aperture ring. Only if it did NOT work with the Sixtub, then I would go to the deeper Sixtif.
Without testing (as above) then I would use the deeper Sixtif with the 40.
In practice the recessed lens panels are sometimes difficult to use. This is because of limited access to the lens for your fingers. More so if you have to reach INTO the recessed lens panel to reach the aperture ring. I recall an accessory handle that was somehow attached to the aperture ring of the lens, so you could turn the aperture ring w/o reaching into the deep recessed lens panels.
35mm half frame used to be a popular format. Slightly smaller camera, and you got 72 shots out of a roll of 36 exposure 35mm film. It was the point and shoot of the day. And as most people did not print larger than 5x7, the smaller negative format was not a problem.