Recently I've come across sentences that doesn't have "one" in it and it looks like odd to me because I'm used to say "which one...?" The sentences must be correct because they are from a grammar.
Which do you prefer - meat or fish?
You can have tea or coffee - which do you prefer?
To me,"which one" would be more correct. I don't know. Perhaps sometimes "which" and "which one" can be interchangeable.