Jim, what kind of photography do you do with your M? Since it takes you a month for 36 exposures, do you make casual photographs throughout the month, or do you find a subject and go through a roll? Also, have you used a medium format camera before?
Unless you have money to burn, I recommend that you buy something cheap, like a Yashica TLR, and just try it out. See how you like the larger negative, and see how you like working with a slower lens. A TLR is actually a light camera, and it's not hard to use them in dim light. The Yashica lenses are really quite good, and I like the images I've made with mine. I also use a Holga, as it's weight is only 8oz, and the images are better than you'd expect. Tape up the light leak holes, and it's really a good camera. And of course, who knows, you may like them!