Now I own both bodies and I am a long time M user.
Before I have allways said best thing to use M lenses is the M body.
But I am not sure any more.
I think 35mm and 50mm is great on the M, but for wide angle and for lenses 75mm and longer the EVF of the SL has advantages for framing and in case of 90/135mm also for focusing.
Another advantage is the better exposure metering of the SL.
It is the first time I wonder if I should not just use the SL for both my M lenses and the SL-Zoom and get rid of the M.
One thing I might miss sometimes seeing around the frame in the viewfinder. And the rangefinder has the advantage to see the whole frame during focusing (and not just the magnified area).
In regards of IQ its a wash for me, both do fine for me.