to be fair to @DAiDEViL, Mandzukic at the same age as Zaza is now was playing for Dinamo Zagreb and scored 14 goals in 24 games. the season after, he went to VW and scored 8 goals in 24 games and 12 goals in 32 games prior to his move to Bayern. before Bayern bought him he wasnt really a world beater either. and this season he has 12 goals (3 penalties) in 28 games.
Zaza on the other hand, who i find very underrated here, has already 2 Serie A seasons in which he scored 9 and 11 goals and before that scored 18 in Serie B. he is still 3 years younger than Mandzukic was when Bayern bought him.
not really fair to compare them, as they are at different stages of their careers, but if Zaza and Mandzukic are both 23/24 now, statistically they are pretty even, id even give Zaza the edge here...