Which song do you prefer, Popular from Wicked or I Could've Dancedall Night from My Fair Lady?

It depends, if the Broadway night is just for fun then you could do Popular. But the song is overdone and it mostly "fluff" compared to other Broadway material. I Could have Danced all Night is a clic Broadway tune. It requires a certain amount of mastery and charisma to preform. It also doesn't hurt to try something new. Plus, if you do decide to act on stage then you have a new song to add to your repertoire when auditions roll around. And I would not suggest using Wicked songs for auditions.