I pulled this up from a youtube comment from the thread Chiko mentioned (I'm guessing this comment was made by Chiko herself), not the absolute definition but is a good way of explaining the difference:
Bisexuality and Pansexuality are very similar in a lot of regards, and it sort of comes down to what a person is comfortable with, personally I like the definition that, like you said, "Pans don't have a pref" whereas with bisexuality I can say "Oh I prefer girls over boys and/or I like x in boys, z in nonbinary people but y in girls." but it comes down to who uses the word and how they like to define it, however, don't go saying bisexual people can't be attracted to people outside of the gender norm