For those interested, I made the decision and went for the Leica. I found a good deal on a used one and will use it in parallel with my old camera while I decide which one to keep. Its looking like we already have a winner. Now I have to get out and produce some decent work to pay for all the expense!!
While the specific circumstances that prompted this thread have been resolved (and a huge congratulations to
@steveash on your purchase!!!), I think the general question is still an interesting one.
Most of the replies have tended to fall into the category of "ergonomics matter because you will shoot more with a camera that feels right ergonomically". Ergonomics can relate to nothing more than "the way it feels in my hand". It can also include how easy it is to access the controls you need either through button layout or menu navigation. All of this is really important and I don't want to downplay it.
My counterpoint would be: does the "ergonomically best" camera allow you to capture the kinds of images that you want to capture?
For example, I used a Leica M9, MM, and M(240) for years. Once my son started getting older and moving around more, I found that manual focus with a rangefinder was vastly inferior to having AF. While I loved the feel of the Leicas, they weren't the right tools for what I then found myself shooting most of the time.
Another example: last year I bought an underwater P&S to take pics of my son in the swimming pool (the positively-reviewed Olympus Tough TG-5). It can shoot RAW and has an f/2 lens. Ergonomically, it is a nightmare to use. Nothing about it is intuitive, from button placement to menus. I absolutely loathe the experience of using it. Adding insult to injury, its files aren't even close in IQ to what I can get with my other cameras. Yet I still use it, because it can do something that none of my other cameras can.
There are certainly cases where the "bells and whistles" of one camera are nothing more than fluff. But there are also cases where an ergonomically inferior camera can do things that a better camera (in an ergonomic sense) cannot.
Don't cut off your nose to spite your face--if a certain camera (or camera system) has features that are important for the images you want to create, ergonomics sometimes has to take a back-seat to the realities of being able to capture the images you desire.
) along with two rolls through a Nikon F2SB-this is one of the ones that requires you to twiddle the aperture ring when you mount a new lens. I find F2s on the whole to be nice to use, but they're not overly ergonomic either. 
