What are the major differences between these two programs, besides the difference in price? Presumably there must be some difference, to justify a 900 dollar price gap, but I haven't been able to find out what it is.

Also, how does Logic Pro's handling of MIDI editing compare to Soundtrack's?

Soundtrack Pro has no MIDI capabilities, nor a CD mastering program and has a lot less plugins.

Doesn't Garageband allow you to edit MIDI files?

Logic is Apple's "Protools". It's a multi track audio/midi recording, editing, mastering program. Geared more towards music production than film audio.
Soundtrack is a lighter version of just audio editing.

Also, how does Logic Pro's handling of MIDI editing compare to Soundtrack's?

it exists.

Thank you all for all of the great answers.