I think there can be a really good debate about which player is actually better, Mbappé or Haaland. Based on what we have seen, so far, I think we can easily say that Erling Haaland can definitely be described as a better player compared to Mbappé. And let's not forget that English Premier League is definitely a lot harder compared to Ligue 1. And that is a big reason why I think the current performance of Haaland is better compared to Mbappé. But both are definitely up there as the two most important and probably best young players right now. It won't be very wrong to expect them to be the replacement for Lionel Messi and Cristiano Ronaldo.
Another thing is, I don't think personal performance actually means anything if the team is not winning. Haaland with Manchester City has been able to win a lot of things so far. The same cannot be said with Mbappé with PSG.
You're right Haaland has proved himself in several leagues apart from the EPL before joining City whereas Mbappè's best performance is at his country France both club side and national team but Halaand has proved his a very good finisher in four different leagues and i believe if he had quality players to back him up like Mbappe's got at France their country would've been one of the strongest so far and his performance for country side would be outstanding.
Okay let's come down to their performances last season, Mbappé scored about 7 goals in the Champions league with no assist, 29 goals with 5 assists in the Ligue 1, and 5 goals in the French cup competition making it about 41 goals with only 5 assist in all competitions last season whereas Haaland stats is more impressive in the Champions league he got about 12 goals, 36 goals in the EPL with 8 assist and 1 in the EFl cup making it about 49 goals with 8 assists in all competitions even won the treble and player of the year in both Champions League and EPL, so who woukd you say is more better? The difference is clear.