Thanks folks for your helpful replies.
With respect to diesel vs gas, well, that's a choice rather than a need. With four diesel tractors already in the fleet I'd like to reduce my jerry can fleet! (Yes, four diesels... one of which is not a Kubota so it doesn't get mentioned much here.)
From experience I know that 60" is as wide as I want to go. I have both a 60" and a 54" deck for the B-series tractors so I have an idea of how well they cut, and, importantly, how much power it takes to drive them effectively. Even at that I don't run the 60" on the tractor at the airport because, well, because I can't go as tight to airplanes with it and because I can't go as fast with it as with the 54". I've tried them both and was certain the 60" deck would be faster. Nope, my intuition was wrong... again!
I'm liking the idea of the Country Clipper joystick... that gets me back into a gas mower again, but an XLT with the 26hp Kohler and a 52" or 60" deck would seem a decent compromise.
The one thing I have to say about the MMM's for the Kubota B-series tractors is they are built like absolute tanks. I suspect I will be disappointed with anything less than that level of robustness.