Obviously Negredo is a very interesting player. He scores goals on a regular basis (not just a one-season wonder), he scores them in a relatively mediocre side in La Liga, he's fysically strong, and must have a bond with already signed Navas.
He is however not the one big name player we are after. As for a name he's no Falcao, no Cavani and probably no Lewandowski. But that doesn't mean he can turn into a great player for us. Yes, he can fail. So can any of the big names tbh.
I'm just not too sure if he brings so many different options than with Aguerro and Dzeko. As for Guidetti, he is a big talanted striker who, however, is not fit and has missed to over a year of competative football. When he's fit and challenging, and he's better than any of Aguerro, Dzeko and whoever we bring in, he will play. If Negredo is a risk for some, Guidetti is an even bigger one right now.