Anyway, I think May had the biggest breakthrough in a way because she was the first female to actually have an intresting sidequest, but I think Dawn overall had a bigger impact on the company in general. Since, she became the first female to approach Ash's level of stardom and that was unheard of even with May's introduction.
Its pretty much the same thing. Dawn got more promotion because they had practiced with May prior and wanted to do a co-star thing again.
I find it interesting though that May got more Contests overall than Dawn did. May got 16 Contests and two Grand Festivals, whereas Dawn only got 11 Contests and 1 Grand Festival.
Since I don't think Dawn will stay after DP, it seems like May got more Contests than Dawn overall, even if Dawn got to star in more episodes.