Well, May is pretty strong, but it was a shame she wanted to be a coordinator. Dawn however, is a noob like Ash in the first season(when Pikachu hated Ash but became best friends after a few episodes...like Piplup did to Dawn) May had experience in battling(since her dad is a gym leader) but Dawn had no experience in battling and catching Pokemon(like Ash) So who knows? Things like this happen a lot! I mean, look at Ash. He was a noob at the start, but he went through 3 different regions!(including Sinnoh) Plus, if Game Freak would design ANOTHER region, then there is a possbility that Dawn will eventually have stronger pokemon...and she may even have that Mesprit!
I conclude that Dawn is better than May(even though Misty is waaaaaay better)