I would add that getting rid of a Nikon rf system in order to upgrade a Leica M system is, in the long run, the surest way to burn through a bank account. Not sure that "I think I will concentrate on Leica" is the actual economy move here, but I imagine you realize that.
Yes, there are more lens choices for the Leica, but that's just more money, and how many choices do you need, and is there really much, if anything, you can't do with the SP and the nice lenses available for it. I doubt it.
Stephen did a nice CLA on my S2. Nice cameras which are exactly that, nice.
Good luck whatever you do.
I can understand your reasoning, and realize that both systems will be expensive. However, I am bonded to my Leica, especially as I got it from someone I know. As far as burning through a bank account, I have the sales of my current RF kit (The Nikon, the Bessa R2, the 35mm f/2.5 Color Skopar, the Summarit) to work with and I will try hard not to exceed that. Especially since I'd actually like to keep the Bessa if I can. I also have an M2, which means that I only have 3 framelines. This is intentional. I like the 35/50/90 trinity (even if my 90 is an 85) and use it as an LBA limiter.
I may eventually get something MUCH wider than 35, to make using the external finder worth it. But I want to get to a 35/50/90 setup that becomes more or less permanent.
The 35s I'm considering are the Voigtlander Noktons, the 35mm f/1.4 and the 35mm f/1.2. For a 50, I am considering the Nokton 50 1.1, though the 1.5 and Canon RF lenses have caught my eye. I would also love to have a 50 Lux, but I'm not holding my breath that I can get a good one for within my budget. As far as the 85..I have a Serenar 85/2 and it's wonderful. I feel no need to change it, unless I can find the same sort of optics with a shorter focus throw.
The Nikon SP is at the top of the list in the CFC (aka cool factor category), outdoing any M Leica.
Cool Factor, the Nikon wins hands down. Rarer, more functional, more fully featured than an M2 or any unmetered M. But It'll all be about the shooting experience, lens inventory, and of course, the images.
I am intentionally trying to concentrate, "Distill" if you will, my current camera inventory into a smaller collection of nicer things with the same approximate net worth.