That is not that easy. You could argue about which one is better, Ronaldo or Messi, but you can't really argue the adaptability since Messi only played for Barcelona whereas Ronaldo played for Porto when he was young, then United, then Madrid, then Juventus and now United again and under so many different managers as well. All in all I would say that it is clearly a proof that Ronaldo could play anywhere and be good.
Ronaldo has more experience than Messi because in his career, he plays for different clubs in different leagues from Premier League, La Liga to Serie A. He leaves big records in all clubs and leagues he joined so far. I am not Ronaldo fan but I would prefer to see Ronaldo has a few seasons in Bundesliga or Ligue 1 to complete his career in all top 5 leagues in Europe. However, with his high ambition, I think if he joins those leagues, he will join PSG or Bayern Munich only. Because with them, he can win titles.
Messi is unknown on that front, he only played for Barcelona which is a great thing for loyalty and all but then he needs to prove himself to be worthy at PSG as well, he is still a great player no matter what happens but I believe that between getting older and playing in a new team, things do not look like it could be simple for him.
Messi does not adapt so good in PSG but I don't think he is responsible for all. Coach has his responsibility as well. You see despite of successful adaptation to leagues, clubs, Ronaldo has difficulties sometimes and in general, one player can not improve a club. If you put Ronaldo into Barcelona squad, do you think he can improve them a lot and instantly. I guess he can not do it.