I like them both equally.
I have owned and used a Leica II, and now a Leica IIIf RD ST. Since I mostly shoot black and white film and process it and scan it myself, the glass available for screw mount is more than adequate. Right now my IIIf is wedded to a collapsible Summicron and I do not see that changing any time soon. From portraits to landscapes I am happy very happy with that lens. Youxin did miracles for the IIIf body and it is super smooth. With the SBOOI finder, I do not feel like I am making any compromises with the screw mount camera.
Ah, but the M. I have owned and used two M4s, an M2, a digital M-P, and an M3. I have used the very best and newest glass like the 50 Summilux ASPH, 21mm Super-Elmar ASPH, and 35 Summicron ASPH, and I have used rigid Summicrons, DR Summicrons, 90mm 2.8 Elmarits, and 21mm Super-Angulons. The newer glass is lovely on the digital M, but a bit silly for black and white film. I let go of most of the newer lenses and now I use a DS M3 from 1956 with a Rigid Summicron and the newer E46 90mm Elmarit.
The thing about the LTM is that trimming film is annoying. The thing about the M is that by comparison it is a whale. Both are are beautiful machines both to use and to behold.
If I could only keep one, though, it would be the IIIf RD ST.