I've seen some new slot cars with such fine guage wires to be little thicker than a human hair. I've purchased a few used models with such thick (inflexible) wires the guide hardly rotated.
I know that sometimes wires are stiffer because of age setting into the insulation and stiffening or different materials are used the insulator. Ignoring that for the moment - what is your preferred gauge of wire and do you like it a little on the stiff or flexible side?