Don't think Maddison is going to bulk up much nor get a ton better than he is now. He is however a better passer than Grealish and if you are looking to get the best out of Auba next season he's more equipped to do that and has a history of finding Vardy who makes similar runs to Auba.
Grealish would be more of a Sanchez type for us in that he does a lot on his own, like Claud says he drives things. MAddison is a bit of an Odegaard+ in that he can control tempo, make the killer passes but he also has more goal threat than the Norweigan. Maddison is a great set piece taker too.