Mbappe is superior technicality, both are outstanding physically, Haaland is the better finisher with more variations in goals. I doubt either will get to prime Messi-Ronaldo levels though, technically there would be better footballers than the both of them, you could also factor in consistency, can they do 12years of being the best two? I feel in the current era Messi is the most gifted player, only Ronaldo could keep up, neither Mbappe nor Haaland has the brilliance of Messi, the likes of Sancho, Foden and other youngsters could run them closely and look better some years.