Mahrez was a more noted goalscorer, having been arguably the most valuable player in a team that managed to win a title. He's also more a winger who generally stays on the wing.
Jack Grealish........ has basically had two good seasons in the Prem, for a team that finished 17th and 11th. He also often picks the ball up deep and in central positions, holding onto it for a lifetime. Something that will never, ever wash in a team coached by Guardiola, and filled with the likes of De Bruyne, Foden, Bernardo, Gundogan, Fernandinho, etc, etc. Players who are better than he is, basically.