Her old design seems to have more of a better connection to me, in a way, since it was used in anime and whatnot. Back when Advanced series was still around, May used to be one of my favorite characters, and being able to play as her in-game exactly sort of added to the touch, if you know what I mean.
Though, I'm definitely fond of her new outfit; definitely an improvement over old. If you ask me, it's very modern and stylish. Hoenn is a tropical region, and is meant to be hot in most of its towns, routes, and cities; wearing light clothing like she's doing in ORAS does make sense, in a way. Only thing I dislike, though, is lack of gloves. In her older design, she had them, and they were actually pretty cool-looking!