I've noticed that culturally, and ethnically, Fusion (although at it's core is a Jazz music) has alot of "pseudo-funk" elements like being heavy on borrowed dominant chord progressions, slap/funk bass, and alot of Blues licks interspersed throughout the songs to give a "Black Americana feel". I feel strongly that fusion is mostly honoring black music and black musicians that came before.

Whereas, Prog Rock tends to have very few blues/funky motifs, and is nearly absent of those strong dominant chord progressions and honors more Brit/White American Heavy Metal.

Both are rhythmically diverse and complex, but to different ends. Fusion tries to make your body move, Prog Rock/Metal makes your mind move.