I would pick age, if that was an option. And obviously acting talent. I don't care what race they case as Ariel, for example. Like I wouldn't care at all if they cast a black actress as Ariel. Physical resemblance isn't necessary. But I do care that they cast an actual 16 year old to play a 16 year old. Or at least some one close to that age. When you have 20 some things playing high schoolers then it gets real creepy

An addendum to my previous statement, physical resemblance does matter if the princess is a POC. They should at least be of the same race/ethnicity.

Acting talent is really matters for pick any actress for any role in a movie in general. Physical appearance also quite important to make sure she resembles the princess enough, but not necessarily have to looks very alike (ex: you wouldn't want Caucasian actress to be casted as Mulan, so they should cast a Chinese actress for that role to make sure the movie would be as accurate as the original was)

and acting talent both go hand in hand. Like Emma Watson was perfect fit in terms of looks but was really bad talent.