Uncle Bob,
Correctomundo on wheel dia, but I suspect we were getting at the
fact that thinner width tires are going to be hard/impossible to find
in 20" dia. 18s, even 17s will be easier to find in snow tire size and
the resulting preferred narrower width.

Hard to diagnose the OPs real "needs" from afar, as to his real
usage and potential conditions. Stock 20s will not be fun in any
real snow/incline & descent conditons, imo.
BR,mD