Which phrase is correct, "dependent on" or "dependent upon"?

They are equivalent; upon is just a more formal term for on, and it is especially used in abstract senses.

The copy of the NOAD I can access on my Mac via the Dictionary app reports the following note about the usage of upon.

The preposition upon has the same core meaning as the preposition on. Upon is sometimes more formal than on, however, and is preferred in the phrases once upon a time and upon my word, and in uses such as row upon row of seats and Christmas is almost upon us.

The same note is readable on the Lexico website.

based on this NGram, "dependent on" seems slightly ahead

here's the same image with some smoothing