I have the M-D, prefer it to the M10, and I have the SL.
SL: This is my technical camera with ultimate versatility. I have been using it lately mostly for negative copy, table top product work, and ultra-wide work. It can use every lens I own, regardless of mount when using adapters, but I mostly use SL or R lenses on it. I almost never use M lenses on it. It is supremely capable for anything I will ever want to do photographically.
M-D: My "grab and go" camera. I've mostly used the Summicron-M 50mm and Summilux 35mm v2 lenses in the past half year. People, events, general photography, etc. Ultra-simple to use: when I use it, I'm restored to my old favorite way of working, just concentrating on the subject, exposure, and focus. Nothing else. It's smaller and lighter than the SL, of course, and a better choice when I hop on the bicycle or motorcycle since I can carry just it and one lens in a very tight small protective bag stuffed into my backpack or tank bag.
I am completely satisfied with these two cameras. All of my other digital cameras are either gone already or for sale, and I'm not looking to buy any others. I'll be working with them for the foreseeable future.
Choosing between them ... For me, it depends entirely on what kind of photography I think I'll be doing.
- If the former work, I'd buy the SL in a moment. I love playing with unusual kinds of film work and the SL is now my primary means to digitize 6x6cm negatives.
- If the latter, the M-D is all I need.
I'd then wait until I could afford it and buy the other.
G