Zaccheroni won a scudetto, Del Neri didn't (both started coaching at the same time).
Zaccheroni has bigger European experience . Del Neri has only 14 European matches in 24 years of his coaching career. Only 6 of those are in CL and his record there is 0-1-5.
Zaccheroni already coached big teams (Juve, Milan, Inter). Del Neri didn't. The biggest two teams he coached were Porto and Roma. Porto fired him even before his debut and with Roma he didn't even finish the season (replaced in March).
And I really don't rate Zaccheroni.
As for the rest, they're almost the same. Both had some successful and bad spells. We know about Zaccheroni's bad spells but Del Neri also has them. He was fired by Palermo, Roma and Porto and he relegated Chievo in 2007.
Seriously, we can't be fooled by the few good episodes he had with mid-table teams. If he was a good coach, he'd have already proven it, especially since in his long career he was given a chance to make a next step from a small team to teams like Porto and Roma.