Rather than a question of connotation, it is more of a question of versatility. MLK's "I Have a Dream" speech includes this clause:
"I have a dream that my four little children will one day live in a nation where they will not be judged..."
Obviously, it would not work to say "my four little children will once live..." As far as meaning, the two phrases you have are identical. The difference between "once" and "one day" is that "one day" can express both past and future events while "once" can only express the former.