I haven't listened to much of Ozzy's solo stuff, but I hear it wasn't as good as what he did in his Sabbath days. A shame, too, because the first six albums were phenomenal.
As for Dio, excellent stuff. He's one of the most underrated singers in all of metal, and it's a shame that more people don't know about him. His Holy Diver album is awesome, and though he's had a few low points in his career...well, so has everyone.
So, I'd say they're two different styles and equally good.