I think the big difference between Sonny and Fredo as far as being Don,is that Sonny turned out to be a bad pick in hindsight because of his temper and the emotionally driven decisions he made after taking the reins, but before that,he had the makings to be successful,and was a good choice.
Fredo,on the other hand,was already out of the running,because he was never seen as anything but weak and stupid.
Sonny had the potential to run the Fanily right,whereas Fredo had nothing from the get-go.