TLDR Iggy is still better finisher, Alvaro might suit us better.
Higuain is a better scorer atm, but he became somehow limited. Napoli Iggy was able to run at the defenders, finish crosses, dribble, etc. Nowadays Gonzalo is a pure goal-scoring machine. When he faces opponents goal, and you give him the ball - he is going to put it into the net more often than not. His positioning is amazing, but he can't pick up the ball and just push it forward, he rarely finishes crosses, he lost his speed.
But there is no vertical depth in our attack now. Both Gonzalo and Paulo operate in same part of the field, and it's easy for opponents just clutter the lane and make for any minor positioning errors they make. While someone like Alvaro forces defense to respect his speed, ability to beat opponent one on one and to finish the cross.