Re: Final ranking
I like Bon more than Brian, but I like them both. At one time Brian was technically better in the studio, but charisma aside Bon really was always the better live singer. Even at his height Brian had trouble sustaining his delivery through live performances. Not sure if it was an issue from the very, very beginning, but if it wasn't then the issues started showing up live long before he declined in the studio.
That said, I think that on the studio albums he only sounds really bad on The Razor's Edge (although 2/3 of the record is still very good overall) and Ballbreaker. He had throat surgery after that, and was at least decent in the studio on the next couple albums.
I only really listened to Rock Or Bust once or twice, and haven't heard the new one, so I can't weigh in on those.