Rationalizing isn't an inherently pejorative proposition. Of course, you can make it so. Logic can be a tricky thing when dealing with desire. The mind is a marvelously flexible thing. It is largely a matter of being honest with yourself when considering your motivations. For example, I admire Robert's forthrightness. I am happy to entertain your dispassionate logic for moving to an M10 with its attendant cost from its immediate predecessor. Perhaps it will be of some benefit to Bill as he weighs his decision.
You're posing everything about this decision as a matter solely of "want."
In my purchase decision process, "want," "need," the expected advantages of the purchase, and the fit of particular gear for my intended purpose must be in the proper balance to make me press the Buy button.
This is what I mean by "reasoning" rather than "rationalizing". Rationalizing isn't necessarily pejorative, but it is second intent ... You want something and you contrive reasons to make it sensible in the usage posed on this thread.
That's not how I consider purchases of camera equipment. When I see something new that looks interesting, I look at how it poses an advantage to what I am currently using and what new things it might enable me to do that I haven't been able to achieve with consistency using what I have. If the thing enables something I haven't done before, I think about whether I want to do it, why, and what the chances are that I'll actually do it.
Sometimes these notions of intent and purpose shift as I start to do something too, and then the basis of what I've purchased changes. That's the point were I sometimes rationalize, "... Well, I thought I'd do some motion capture work, but I never really get to it because I have other things I'm doing that are more important to me. However, I like that equipment and there's no cost to keeping it, so I'll continue to keep it and use it for what I'm doing." Etc.
I looked at the M10. It came out after I already had the M-D, so when I looked at it I had the M-D with me to compare it specifically. My evaluation at that time was that the M-D was the better camera for me ... It met my needs and felt better in my hand. The latter is still true, but I'm considering a rather big change in my equipment holdings to accommodate what I'm actually doing now, three years on from getting the M-D and the SL. The M10's additional features would be more useful to me now given how I'm planning to change what I've got in the equipment cabinet.
The primary thrust of my advice is to let go of all the over-analysis and desire oriented vacillation, look at what you do and what equipment would do it best, and then make a simple decision based upon advantages and what you can afford. It's really not that difficult to do, or that difficult to understand the advice.
G