From what i have seen on wikipedia predicted sales put MJ at 3x more records than cliff, despite not making much music of note recently.
http://en.wikipedia.org/wiki/List_of_best-selling_music_artists#1_billion_records_or_more
(The link would put MJ 3rd behind the Beatles and Elvis Presley) - though as it says in the intro, very difficult to know any exact amounts
That link also had Wet Wet Wet on the same number of sales as Led Zeppelin and Pink Floyd so I don't think it's accurate.