I wasent even alive during the music box era and niether were a lot of the people in this thread throwing around facts so i dont think anyone can judge this unless you old as hell
21 sold better in comparison to each era's respective sales climate, and Rolling In the Deep, Someone Like You, and Set Fire to the Rain are more recognizable than any of those songs from Mariah's album.
MB may have a sold a bit more (a million or two) but 21's sales (27/28 million) are insane in today's climate! Someone Like You is definitely up there with Hero and Without You. Rolling in the Deep is perhaps the biggest hit of this decade, so far.
