Rosalina's image is still a variation of the standard design set by Peach, but the final result is not as faithful to the original model as the concept art created during her development. Perhaps someone on the team realized that they didn't want to create another Daisy (or really, another Peach). Either way, Rosalina lost the puffy frills and bright color scheme and her dress started to reflect her more mysterious personality. It's also worth noting that her nails are tinted purple; a pretty insignificant detail that sets her further apart from the other princesses in the series.
But which one is better: Peach or Rosalina?