Hi Seth, for years I ran my locos on a mix of both types of M-track with no problems.

Some locos would slow down on the 3 rail track due to increased friction, but that was the only effect I would see.

I would imagine that the bigger the surfaces touching each other, the bigger the surface on the slider will show wear. It all depends of course on how easily the slider veers up and down (spring action weak or strong).
Ever since Maerklin switched to point contacts (in the late 1950s), the sliders have become longer, obviously to obtain good contact with the (small) point contacts, but also to make sure the slider does not touch the rails on turnouts.
K-rail has wider point contacts than the newer C-rail. Obviously, Maerklin's aim is that the track looks more "2-rail" than "3-rail", to please the 2-rail "fundamentalists" among the 3-railers.

Willy