Lukaku scores more goals because he plays far more minutes. He's also the main focal point by far in the teams he plays for, which won't be the case at United where there are other quality attackers alongside him. I think people who expect him to be more clinical than Zlatan was for us are going to be disappointed. Overall our attack is still weaker than it was last season.They are at the same age, but the lowest amount of goals Lukaku scored since 5 seasons is 16, playing for inferior teams like WBA and Everton. Morata had his best season at "just" 20.
Lukaku has been an efficient and consistent goalscorer in all competitions he has taken part in. I'd preferred Morata as the more rounded one as I've said but in terms of a pure finisher I'd take Lukaku.
Mind boggles why some are not happy if this transfer goes through. He's as quality striker who is on the market as any.