From what I've seen of Hernandez, I would agree with Red. I would need to see a lot more continuity and production out of him to be able to make an insightful prediction about how he'll turn out. Obviously has some quality, though.
I've seen much more of Jovetic over the years, and I do think that he can be a solid contributor when it comes to leadership. With guys like Buffon, Pirlo, Chiello, Marchisio, and others, we wouldn't need him to step into a primary leadership role, either - he can just be another guy who adds to those qualities in the locker room.
I do think that he can become a superstar. I won't say that he can be like ADP, that would be sacrilege at this point, but I think he is very, very talented - and I do think he has a lot of the same characteristics in his game (also, both had knee injuries, and have a similar 'weakness', really: pace isn't their strength).
I wouldn't worry too much about Jovetic going the route of Giovinco, either. Seba's physicality imposes certain restrictions on his game that Jovetic will never have to deal with on a day to day basis. There are also environmental factors at the club which seem to stress Giovinco out quite a bit; probably just a result of the mess he experienced the first time he was here, and the fact that he's Juventino and just wants to help the club so much.
The environment at Fiorentina is much different, but Jovetic has performed well in limited European action, he's been a mainstay for his NT, and I think he could handle the pressure that would come with joining a big club like Juve.
Jovetic is a few levels ahead of Giovinco in my opinion, and I think he'd be worth the 25-30M investment.