Quote Originally Posted by kwmullet

cost
LSM lenses on average tend to cost less than M mount lenses.
Non-Leicas that have the mount I want seem to be a good way to get
into the market.
The M was introduced around 1954, I believe, with a slight overlap of lens availability between the two mounting systems. Generally, however, the thread mount lenses are of older designs, some of which have a strong aesthetic following, even if they were less sharp or contrasty than newer designs. There are, however, LTM-M adapters available, allowing the use of the older lenses on Ms.

Quote Originally Posted by kwmullet
Rear element clearance
Some wide angle lenses and collapsible lenses require more clearance "in the box" than some third-larty LSM- and M-mount cameras can provide.
Yep. There are, indeed, compatibility issues between some of the Leica clones and earlier wide-angle lenses. A couple of the Leica "reference" sites provide a run-down on which lens won't work with which camera. Also, some of the non-Leica M-mount cameras have a slightly different mount-to-film measurement that needs some caution.

Quote Originally Posted by kwmullet
two or one finder?
It looks like cameras contemporary to the Leica III series have two finders: one rangefinder to focus, and a separate viewfinder to frame the image. This is the arrangemeng I have on my Crown Graphic. I guess preferences on this would be a thread in itself. With two finders, you have to switch back and forth every time a subject moves and you have to re-focus. One finder with an RF is still a compromise, since you have to move the focus spot to the point of focus, then move the frame back to where you want it.
True. There is also an issue of slight variation in the triangulation of the rangefinder when re-composing in critical focus situations when shooting at f/1.4 or f/0. Essentially, if you rotate or turn the camera on its center (the lens axis) when re-composing, you change the distance to the subject slightly, thus changing what's in focus. So, you either have to "slide" the camera to re-compose, or make a slight adjustment in the focus through experience.

There is also an issue of rangefinder accuracy between Leica Ms and the clones. The clones mostly have shorter rangefinder base lengths (the distance between the two rangefinder windows), making the rangefinder slightly less accurate.

Quote Originally Posted by kwmullet
Single frame, multiple frame, or accessory viewfinder?
I get the impression that some RF cameras have a mechanism for switching the framing lines in the viewfinder between common focal lengths like 35,50,90. On older cameras, what I'm reading is that this yields a viewfinder that doesn't age quite as well, and is a bit dimmer than single frame viewfinders. If I want to frame more accurately with non-normal lenses, though, in a brighter, single-frame viewfinder, it seems that I need to use an accessory finder.
I've only used the M6TTL, which has lens-linked finder frames, but its finder is plenty bright for even low-light shooting. Older cameras often need a good CLA, including a cleaning of the viewfinder and rangefinder elements, as over time, these get grunged by atmospheric contamination.

Note, too, that there are variations between earlier M models as to which frameline sets were included for which lens focal lengths. The popular M3, for example, included frame lines for 50, 90 and 135 lenses, but not 35 or 28. Thus, auxiliary finders (hot-shoe mounted) are necessary for those lenses. Later Ms offer different viewfinder magnification ratios, which also affects the frameline set included. The choice essentially pivots around whether you want a 135 frame or the 28 frame.

Quote Originally Posted by kwmullet
Resolving Power/Sharpness
From what samples I've seen, I was starting to get the imression that older screw-mount/LSM lenses are generally less sharp than their M-mount counterparts. Just recently, though, I've seen what I thougth were some nicely sharp/contrasty images shot with Jupiter LSM lenses with a Russian (Soviet?) knock-off Leica clone. Is the issue that older lenses are just more likely to have accumulated more abuse and have the potential to be either WAY less sharp or JUST as sharp as contemporary lenses, depending on the particular lens? How about the concept of the LSM mount itself? Wouldn't screw mount provide for some minimal variance WRT exactly where the lens stops on the thread as opposed to the (seemingly) more precise M mount? Does M provide more accurate and predictable focus than LSM?
In many cases, I think the differences can be rather subjective. Later M lenses included design improvements that increased resolution and contrast, and some of the latest designs include aspheric elements that provide even better correction for certain optical abberations. Wear within the focusing mount can certainly affect sharpness, but the old 50mm DR Summicron is still one of the sharpest 50s around.

Quote Originally Posted by kwmullet
Knob, Single-throw or Multiple-throw?
Maybe it's the way I shot when I was growing up, but I just don't get a sense of closure when shooting with my Canon EOS -- No satisfying thumb throw after each shot. Myself, I just can't see shooting with a 35 that has a knob to advance the film. Ideally, I'd like a single-throw lever. I guess that limts the field quite a bit.
Yep. Leica IIIGs and earlier are knob winds, and early M3s used a double throw advance, as there was concern early on about potential film damage resulting from the thumb lever being too fast.

Quote Originally Posted by kwmullet
So, I'd be willing to get an RF with a single frame viewfinder and have the inconvenience of using an accessory finder if it was parallax corrected and the single-frame viewfinder gave me substantially brighter focusing than a multiple-frame viewfinder. Is that the case?

How about the sharpness/contrast of LSM v. M lenses?

What third-party LSM bodies are likely to give enough clearance in the box to use collapsible and ultra wide-angle lenses? How about the Canon 7 or Canon P?

Finally, I'd like to find something with both M and X synch (or at least X synch) so I can use both flashbulbs and electronic flash.

Oh -- and I couldn't care less if it has a meter. In fact, I'd be happy if it didn't. I'd rather just fly by the seat of my pants or use my hand-held spot.
The auxiliary finders have no lens linkage, so they can't be parallax corrected. Each has a set of instructional guidelines for estimating the framing through experience, however. Even with the framelines in the viewfinder, there is an inherent difference in magnification based on distance, so there are guidelines as to whether to use the inside edge of the frameline or the outside edge, depending on subject distance.

In general, I think you'll find the more recent lens designs to be sharper, particularly those lenses with aspheric elements.

Some of the M4s (or perhaps M3s) had both M and X sync connectors, I believe. Later models are X-only, or perhaps triple-X, depending on your subject matter.

I'm not sure about lens compatibility issues with the Russian clones, but my impression is that most of the problems related to specific wide lenses, particularly the 28mm Super Angulon that was offered at one point. If you get a modern body and use modern lenses, I'm not aware of any problems. With earlier models, particularly clones, you have to pay attention to the specific model/lens issues.