I'm not such a van Aert fan anymore, but for me he's still better on the road than van der Poel. Obviously van der Poel is the better allround cyclist if you look at all disciplines. But on the road I see it differently since van Aert is a great climber, too.
I am still very sure van Aert has the bigger pure engine, just based on the cross races, although I don't follow that closely, but what I saw convinced me totally of that.
I think van der Poel doing long solos which van Aert doesn't like to do like that, is a result of character more than of physical traits. Van der Poel races road without thinking much about tactics, he just goes for it, while van Aert, although not always cleverly, tries to save energy here and there and is more careful in spending it.