Uh, for hard abuse, I'd pick either a R3 or R6 over pretty much any other Leica reflex, and certainly over the SL/SL2.
The SLs were complex and vulnerable, and rather prone to failure when used hard - they never reached a M-like reliability, and most press photographers back then either kept to their M's or left the brand for Nikon. The R3 was a considerable improvement on them, rock solid, and the first Leica reflex which saw wide acceptance among hard-working pros (the R4 initially was a setback, but improved to R3 like quality later in the series).
In the long run, the R3/R4 seem rather more subject to wear than the SLs if we go by the state used ones are currently in, but that may be due to the fact that these were seriously used, while SLs often ended as shelf queens quite soon in their life cycle.
Sevo
Uh, no.
Have you ever owned an SL or SL2? I've shot with an SL for several years and it makes my much loved Nikon F feel downright cheap.
The SL/SL2 may be the two most overbuilt SLR cameras ever made. Talk to a service technician like DOn or Sherry, if you won't take my word for it. There are varying figures available for shutter durability and they range between 200,000 and 400,000 exposures. Leica lost money on every SL/SL2 body they ever made, because the fabrication was of such high quality and cost. The baseplate for instance is made from relatively thick stainless steel, which is much more difficult and expensive to machine than brass. If the Leicalflex has an Achilles heel, it is the silly cover on the front of the prism housing, through which rain can easily enter. I put some wax impregnated gaffers tape over mine and that seems to have solved the problem. Leica had hoped to make their money back on lens sales, but that never happened. These were SLR cameras built to the same standard as a Leica M, but they never saw sufficient sales figures to sustain production.
The only other cameras that are even in the same ballpark, as far as build quality goes are probably the Nikon F2 and Canon F-1, but neither of them have the heft or density of an SL. And that is part of the reason why the SL/SL2 were never widely adopted by pro photographers. They were very expensive (at least twice as much and the lenses even more so) and weigh a ton. A Leicaflex SL MOT is big and heavy enough to hide behind, if you should come under fire.
The R3 is a good camera, but it is more along the lines of what we would refer to as a prosumer body in this day and age. It was not designed to take the same level of abuse as the Leicaflex and pro Nikon/Canon bodies could stomach. Same goes for the R4/R5.
It wasn't until the appearance of the R6.2, that Leica produced an SLR of considerable toughness and it was adopted by some pros (Salgado). I also own one, but to be honest I still wouldn't want to put it up against my Nikon F3-P, if reliability was of the upmost importance.
But as far as use by professionals goes the Leica R series never found widespread acceptance in the industry, which is one reason why Leica has been on wobbly legs since the 1960's. The R line was too expensive and not innovative enough, compared to what the Japanese were (are) turning out. The Leicaflex nor the R series was a modern system camera, with the hundreds of accessories you could buy for a Nikon/Canon. Leica also never adopted autofocus or a highly sophisticated autoexposure system like Nikon's 1005 point RGB matrix meter. The R series was also expensive as hell.
The SL/SL2 may have been technologically obsolete at the moment of their introduction, but the one thing you can't say about them is that they failed in the market due to a lack of reliability.
