Compared to the R6, the Leicaflex SL & SL2 are better cameras IMHO.
Pros & cons are:
Much better viewfinder & focusing
Better dampened shutter & mirror
Much shorter shutter lag
Shutter speed dial can be moved with the tip of your index finger
Bigger & heavier = better holding & handling (also a con of course)
Pretty bomb proof - needs a CLA every few decades
If you want a motor drive, then you need a special model and the motor itself is *huge*
Some lenses won't fit on the SL (and ROM lenses not on either - slightly simplified statement here).
Some argue that as an older camera, it's less reliable (I disagree!)
Some think that the "obsolete battery problem" really is a problem (I disagree!)
It's big and heavy
The SL, especially in chrome, has the aesthetics of a 1960's washing machine... ;)
I looked at an early Leicaflex at a camera fair in France several years ago, and am still trying to figure out the "aerial image" that it presents to the user. Different from a "normal" ground glass image - and brighter.
Thanks for your collected thoughts on the R6 shutter lag. I think it would bother me, the feeling that the camera doesn't react immediately when I press the shutter. My Leica RF cameras have spoiled me.
I can't feel the lag in all of the SLR's I have used. So if I can feel the lag in any I would think there is a problem.
The first Leicaflex has the "aerial image" you describe, the SL & SL2 are quite different.
The SL has very fine microprisms over the entire surface (a bit similar to a Nikon H screen), which makes focusing extremely easy anywhere in the field, even the corners.
The SL2 has a more "traditional" looking focusing screen, but still one of the very best I've ever seen for brightness, contrast and the image snapping into focus.
I do slightly prefer the SL's screen, but the SL2 can use some of my favorite lenses which the SL can't (19mm II, 15mm).