Demi has better vocals but Miley is the better singer, if that makes sense. She has a way more appealing voice and it seems she can use her voice to make fun music and do whatever she wants with it whereas Demi is only good at showcasing hers.
Demi has a better voice but Miley is a better performer.
As far as albums, I only really like Can't Be Tamed and the singles from her previous albums. I haven't really listened to her previous albums in full.
I dislike Demi's first album but her second is pure perfection and Unbroken is really good!