どちら・どっち
which (of two)
A と B と、どちらが…ですか
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どちら asks "which one" when there are two choices: 肉と魚と、どちらが好きですか, "which do you prefer, meat or fish?" For three or more, it's どれ (or 何が一番…).
The answer uses のほうが: 魚のほうが好きです.
どっち is the casual version: コーヒーと紅茶、どっちがいい?
どちらでもいいです means "either is fine", and どちらも means "both" (with a negative, "neither").
どちら is also the polite word for "where" and "who": どちらからですか, "where are you from?" That's why you'll hear it from shop staff and on the phone, well beyond comparisons.
Its family is こちら・そちら・あちら, "this way, that way", which you met with ここ and そこ.
Register: どちら is polite; どっち is casual.
Examples
肉と魚と、どちらが好きですか。
Which do you prefer, meat or fish?
電車とバスと、どちらが速いですか。
Which is faster, the train or the bus?
コーヒーと紅茶、どっちがいい?
Coffee or tea, which do you want?
どちらでもいいです。
Either is fine.
赤いのと青いのと、どちらにしますか。
Which will you go with, the red one or the blue one?