Haaland and Mbappe are two different players with different playing styles. I don't think Mbappe is a pure center forward, while Haaland is a pure center forward. Mbappe can dribble like a winger, while Haaland is a typical finisher who tends to wait for a pass. Mbappe can play on the wing and center forward, while Haaland cannot.
They are two different talented players with different styles and skills; they can both finish because they are good at scoring goals.
This comparison is not proper in my opinion because Mbappe is a versatile player that can ply from the wing and also as a striker, while Haaland is a striker that cannot play as a winger; this means that Mbappe can play both wing and strike, while Haaland can only strike.
If I am given chance to chose, I will choose Mbappe because he is a player that can create chance and convert the chances while Gaaland can only convert a chance created.
I don't expect a scenario where Haaland and Mbappe are on the same team in the future. This is not a good choice for any team because after all they are both competitors in the current era. Both are productive center forwards, but I prefer Haaland as a pure center forward over Mbappe.
It will not be a bad decision for any team that will have them both, it will only affect the players individual goals because any of them can score. I love them both because of their goal scoring mentality, if both of them can be in Madrid team, they can win any trophy just that they are both big too good defensively.