A "mint" S2 is probably worth worth several user M3s.....

Which to use as a user camera probably depends on:
1) Ergonomics, do you like the S2 or does the M3 feel better in your hands (and/or give better view and focusing when looked through)?
2) Practicality. While someone mentioned Cosina lenses for the S2, are they really available and affordable? There are many lenses from many brands which can be used on an M3.

My own experience using Contaxes (which the S2 should replicate fairly closely) and Leica Ms puts me firmly in the Leica camp for ergonomic and usability reasons (YMMV).