We have been with kids at both of those ages and I really think three years old is the most magical time. Potty trained or not, this has never made a difference to me. I think it is easier to go in diapers rather than when they are fairly newly trained. One of my DSs didn't finish training until 3.5 and we went when he was 4 and it was horrible. He had difficulty holding it at all and we were forever rushing to a bathroom. My DD was 2.5 on our last trip and she really and truly believes in everything. I nearly cried when she met the princesses for the first time.