Hrm...gotta say Cher for this one. I do like a couple of Dion's songs, but 'You wouldn't know love' is just too epic. xD
Cher. The only two Celine Dion songs I can remotely stand are "Beauty And The Beast" and "My Heart will Go On" (this is because I love the movies "Beauty And The Beast" and "Titanic.") The rest I've heard from Celine make me want to vomit. At least Cher's songs are much better.
If we're comparing vocal talent and ability.. Celine. If we're comparing the two divas overall presence, music catalogs, stage show, etc.. Cher.