If PSG wins, it depends on the impact of both players.Mbappe will win it then, I'm pretty confident. His WC win will be still relevant, add to that the records he is breaking.
And its not like he won't make any impact, if they reach the finals.
Neymar is often the best player of PSG even when he doesn't score himself.
However, i'm a bit doubtful he will be at his best level soon enough after the injury, and to be honest, i could care less about who would win the Ballon d'Or if we win the CL !
I always find hilarious "Barça" fans caring much more about Messi or Cristiano Ronaldo Ballons d'Or than their club victory.