BWV 935 is quite a lot easier according to the henle ratings (well, probably not if you want to play it like gould. But then everything would have to be level 9 I guess)KV 545 has some pretty tough bits (like going up and down the c major scale steadily. It's harder than you would believe.). Also it is overplayed and the BWV 935 is a very beautiful piece and shorter aswell.I vote for BWV 935, but you can try playing KV 545 if you prefer it by that much.