Two different types of players. Mbappe is really a AM whereas Haaland is a CF. I think it is always tough to compare players in different positions, playing in different leagues who are so young. I do personally rate Haaland higher, however this is due to me thinking he is built much better to play in the PL. For me, if Haaland comes in, then it will take him very little time to adapt to the league, Mbappe I think would take a little while.
I see the reverse problem if they went to Spain, where I feel Mbappe would find the transition easier. This is why I feel Mbappe goes to go to Madrid/Barca and Haaland comes to England or maybe Bayern.
However, when I look at the potential destinations in England, I actually see City and Chelsea as his most likely destinations. City due to his Father's connections and because the need a Kun replacement. Chelsea because since Abramovich has been there, all there best teams have had a similar type CF - Drogba/Costa - and the way their current squad is shaping up, they're going to look for the same. Only way I do not see Chelsea being super interested is if Abraham goes crazy and develops a ton in the next 18 months.