Which do you prefer between the Leica M models and the Leica IIIF? I can't decide, as although the M gives a more precise viewfinder framing, the IIIF is a Barnack smaller camera with greater tactile feel.
I have a IIIf and an M2, M4, and M5. I bought the M2 because I always wanted an M, the M4 as an investment, and the M5 because it was the dream camera of my youth.
The IIIf is a delightful travel camera (be sure to have it CLAed) but it suffers from two things: You have to use finders for anything other than 50mm. The other thing is that you either like the look of those old LTM lenses or you don't. I don't, I find them very low contrast. So, I found a few really clean Voigtlander Color Skopars in 21mm, 35mm, and 50mm, and I much prefer that more modern look. You just have to watch, because the glue they used to hold the elements in for that generation of Color Skopars can outgass and etch the glass. So, you want do very thorough inspection any lens you're thinking about buying. There are, of course, also, Canon and Zeiss lenses for the LTMs that are highly thought of.
Oh, and you'll quickly discover that you want to replace the takeup spool in the IIIf with a later version they made that acts more like an M2 spool. Or at least, I did, because I have large fingers and had a hard time getting the original one out of the camera.
Finally, you do have to cut film leaders longer for the Barnacks, but that's kind of not an issue. Leica made a template,
but you can eyeball it just fine.
The Ms have a longer rangerfinder baseline so they are going to be more accurate to focus, generally, especially for longer lenses where that's important. There's also a way bigger stable of lenses available in that mount, though you can - and I do - use LTM lenses on an M body with adapter rings. Most importantly, they have much bigger, and brighter viewfinders than the IIIf. Then again, that only matters for 50 mm lenses on an LTM, since you're using auxilliary finders for all other focal lengths.
I love all these cameras, but if cost is a concern, start with a IIIf and a Color Skopar. As you grow into the system, you can always buy an M body and use the same lenses with adapter rings.
WARNING: A IIIf is a gateway drug into buying way too much Leica equipment. DAMHIKT
The IIIf is still my fave 35mm travel camera - it's small, quiet, and does a bangup job. Here's a couple if scans of prints made IIIf negatives shot on with a 21mm and 50mm Color Skopar respectively in Italy a few months ago: