Compared to the R6, the Leicaflex SL & SL2 are better cameras IMHO.

Pros & cons are:

Pros:
Much better viewfinder & focusing
Better dampened shutter & mirror
Much shorter shutter lag
1/2000
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

Cons:
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...