I would rather Leon win as it protects a character for a future rematch, and gives Ash motivation to push harder (he's already #2 in the world). Winning the Alolan League was a big win for Ash, and he finally obtained a goal he had been thriving for in the anime for 23ish years. At this point, if he defeats the canonically strongest character (unbeatable, damn near untouchable against other champions) he's just going to become another Goku. At this point going forwards it already makes 0 sense for him to lose to anyone below Cynthia, and if he beats Leon they're going to have to power creep every character in the current canon for him to have relevant opponents.
Strictly from a writing sense, it makes 0 fucking sense for him to beat Leon unless they want to retire the character, or say fuck it and go full DBZ and just start going crazy with power levels and invalidate the previous 25 years.
