I have an R3 (not the MOT version) and an R4s. I bought both new, the R3 in 1979 and the R4s in about 1986 when it was on sale with a free motor winder. I much prefer the R3, and bought the R4s for a second film choice and a backup body. The R3 is larger and heftier, and fits my large hands well.
Both have spot metering manual, aperture priority auto exposure spot metering, and aperture priority auto exposure center weighted metering. 99.9% of the time I use spot metering manual exposure, which is in part why I prefer the R3. It has full indication of recommended shutter speed and set shutter speed in the finder. The meter is a needle indicator that runs along a shutter speed scale on the right of the finder, and you can easily interpolate to 1/3 stops. The R4s uses LEDs by a shutter speed scale, and two adjacent LEDs light up for intermediate speeds. The R4s shows only recommended shutter speed in the finder, so you have to remember or double check manual shutter speed settings on the camera top. I shoot a lot on an eye level tripod, and I hate having to turn the camera over to see the set shutter speed.
One other minor feature I love on the R3 is that the frame counter is on the camera back below the wind lever, which again means it's nice to see it on an eye level tripod or without turning the camera over.
The R3 uses a cam driven mirror that slows to a stop as it reaches its upper travel limit, so its impact against the massive body is minimal. I've shot handheld 1/15 second exposures routinely with a 24mm lens on the R3 and gotten consistently good results. The R3 also has noticeably less shutter lag than the R4s. I'm not sure if the R4s mirror is cammed. Has film in it right now, so I can't check.
To make it short, the R3 has some features that make it a major increase in convenience and usability over the R4s for my way of working. My son likes the R3 so much that he saved up and bought a Minolta XE-7 for a system he mostly inherited from grandparents. The R4s is not a dog, it's a very nice and solid camera, and I expect that some might prefer it. The R3 just works like I think.
You'll see all kinds of things about reliability of electronics on the R3 and R4s both. Mine have been used all day in over 100F and below -20F without special protection. (Days out in Big Bend in July and an all day winter carnival on a frozen Minnesota lake.) I've dragged them all over Europe and hiking through the wet Appalachians routinely. I recently replaced foam gaskets on the backs, and my R3 finally needs a little work on the film advance (frame spacing at the beginnings of rolls) after thousands of rolls of film. Those are the only problems ever to come up. Some places on the 'net have info on which serial numbers of R4 to buy, after a modification to the electronics. Might be at cameraquest.com, but I can't recall now.
R lenses are a relative bargain now. I passed on a like new 135 f:2.8 for $200 recently because I already have a 100mm and a 180mm, and because my mom has recently started handing down some of her camera gear, which includes a 135 f:2.8 in superb shape.
Just a note on how she came to buy an R system. I worked for a dealer in 1979 who required me to use an R3 for a week so that I could sell it effectively. My mom is an artist and art teacher and was visiting when I got my Kodachrome 25 slides back from shooting side-by-side comparisons with the R3 and my Canon FTb's and lenses. We threw the slides up on the wall and half way through the viewing she started asking about prices on the Leicas. She made a deal on a used body and bought a 60mm f:2.8 macro, a 28mm, and a 135mm within a week, on an elementary school teacher's salary. The difference was that palpable. My father, who's no particular enthusiast also saw the difference and bought himself a complete Minolta CLE kit for their big trip to Europe after we kids were all out of the nest.
Two other annecdotes about the optics.
A customer at the store I worked in borrowed an R3 and 50 Summicron and shot identical frames of multiple subjects in different lighting with that and a Nikon and 50mm Nikkor. He brought back the Kodachromes for processing, turned them over on the lightbox so that you couldn't see markings, then asked an employee to come over and sort the two rolls of 36 exposures and tell him which were which. A couple of minutes later the result was a single pair of shots misidentified.
In Minneapolis in 1982 or so, camera stores invited their best customers to a Leica sponsored event. Most of these were Nikon shooters, and a few Canon. The main event was a slide show from a photographer who shot stock and magazine assignments with R cameras. About 10 slides in one of the audience asked which polarizer the photographer used to get the color saturation, and whether it needed to be circular polarizing. The answer: "Yes, you need a circular polarizer. I don't own or use a polarizer, but I've heard the B+W is good, and I've been thinking about trying one."
BTW, I still regret not buying an M5 on dealer special, where employees get Leicas at half list, which is 10% below dealer cost. But I was headed to graduate school the next year...
I'll stop now and you can PM me with any further requests for rambling and ranting.
P.S. Mine are chrome R3 and black R4s. I wasn't going on safari and didn't need the matching binocs.