Hi,
Let me put the disclosure first that I am a manufacturer and also owner of the Sanders.
I heard the 20.1 and I own the Sanders 10c. So you know what my response is.
The Sanders has a much cleaner sound, to me, much more detailed ("much more" being relative at 98.9005 to 99.4126 - you get my point). I also see the sanders as faster, snappier and since it is a hybrid, much, much better bass. The bass is as fast as the top so you don't hear the difference between panel and bass speeds. Could the bass be improved? The frequency response range excellent however the control of it could be improved. Having heard and compared active woofers in the same room with the Sanders, a sealed enclosure will improve the bass sonics significantly.
Regardless the bass is much better then the 20 and very respectable in sonic response.
Where the 20 is better is it has a softer sound. I heard both the speakers with the same amps - Merrill Audio VERITAS Monoblocks, driving them. The sources were both tubes however 1 was AMR (Sanders) and the other was Acoustic Research (Magnepans). It is also much slower on the attack and transients. So good for mellower listening, not so good for the rock and roll, fusion jazz, pop. Subjective for the classical and voices. Both are directional (flat planars).
Looks, the 10C is smaller looking and in width. The 10C is also less imposing - there is some transparency with the panels.
Last note, the Sanders 10c requires Bi-amping with an active crossover while the 20's do not.