The contrast is perhaps most noticeable when the preposition phrase is complement of an adjective:
Adjective + on/upon:
(1) The decision was based on/upon firm evidence.
(2) He was bent on/upon vengeance.
(3) It was incumbent on/upon us.
(4) It looks big on you.
(5) He was really sweet on her.
(6) It was hard on her.
"Upon" occurs as a somewhat formal alternant to "on" with the adjectives in (1-3), but not those in (4-6) where "big" and "sweet" are markedly informal. Other adjectives that prefer "on" include "easy", "keen" and "severe".