Son vs Park! These discussions really bring out the sports nerd in me. It's a really interesting case, because Son is clearly the better player individually, but he hasn't won a major trophy. Park on the other hand is very decorated.
If Son had one or two major trophies under his belt then we wouldn't be discussing this right now, as the answer would be obvious. But unless he can change this(I doubt he will win the PL or CL with Spurs), then we might be looking at a debate among Asian football fans for many years to come. It might possibly become as annoying as Ronaldo vs Messi
Whatever happens, I hope that the debate doesn't get dumbed down to Park being remembered as mediocre player who was lucky to play for United in our most successful period. While he might be overly decorated relative to his skills, he was still a very good player.
Back on topic: right now I can't decide who I rate higher among the two. As a United fan I'm biased, but I think it's very even. Had you asked me one year ago then my answer would be Park. Right now I'm leaning slightly towards Son.