Well I like Skyla better in everything but challenge. Better design, better gym, and actually served a purpose where Winnona is just sort of waiting at her gym indefinably.
But I think it is unfair to credit Skyla with a better gym because her gym was on the DS. So I don't think it is fair to give her a point there because her gym probably couldn't have existed on a Gameboy cartridge.
But, in the end, a Gym Leader is a Gym Leader, and exists only for the reason of being a challenging battle, which is something that Skyla epically fails at. Plus, she didn't serve an incredibly big importance to the story, just a single side quest. So my vote goes to Winnona.