I think i didn't point out my insights here. Leveling is more a PvE thing that doesn't give you priority benefits against other players in general. But the Priority login would give an advantage over all players, you can log in earlier because you donated and they didn't. People who didn't donate, still can get every Pokemon to level 100 and still can finish the story, still can get every Pokemon (Pokemon that are now membership only, will be available in later regions).
Edit: You can buy membership in the game from players aswell, so noone who wants to pay reallife money doesn't have to. they just need enough Poke $