I can't fully answer the question myself, but one listen of The Joyous
Isle or Pour le Piano or the etudes should erase any doubt about the
difference.  This is music that is frequently exciting, sometimes
challenging for the listener, sometimes harsh and savage, and requires
virtuoso technique.  And certainly, as you say, the harmonies are much
more advanced.  Also there is the question of classical forms and thematic
development.

Michael Cooper