When it means 'than', na causes an aspirate mutation of the following word, so te changes to the.
I looked up prefer in Merriam-Webster, an American dictionary, and it gave three examples with to and none with any other preposition. Unfortunately it did not explicitly state what prepositions could be used but this suggests that to is standard in American English. Has anyone got any better evidence or knowledge of where or when different prepositions are used?
I'm just now realizing that people in my area (Northeast US) seem to say "I like x more/better than y" instead of using "prefer" when making comparisons - for some reason. I'd never noticed this before but when I was trying to think about which preposition I would personally use after "prefer," I couldn't decide because I so rarely say or hear it. I'm surprised this hasn't given me any trouble translating in either direction.
I don't know why we would stop using "prefer" in direct comparisons when we still do use it in other contexts. Just us Americans being weird again, I guess.