Dybala and Zaza are different players:
Zaza more of pure goalscorer CF,"Vieri style",
while Dybala plays more like SS,he drops back alot,creates chances for himself and the team,goes out wide to the wings.
Dybala suits more the role of being Tevez replacement.
Zaza will be Llorente replacement if can't sign any better WC CF(Cavani/Martinez...) imo.