Is rather than a preposition or a conjunction?

The pair together may be either.

Than is a preposition. It isn't used to join two statements that could each be a sentence in their own rights. Rather is an adverb that must be followed by than; it says that it would be preferable for you to do the first option than the second and has the same effect when put before rather as when put after the verb. So rather than followed by a noun can be a preposition meaning "instead of" (e.g. potatoes rather than rice, stayed rather than going home).

However, the pair "rather than" can also connect an infinitive clause (rather than a gerund), acting as a conjunction as in the example,

"Rather than continue the argument, he walked away."