Body shape wise that they're both proud of, the fact that they have minimal costume changes on tour and that they sing live every time. That's about it.
Kelly has a bigger, fuller instrument whereas Demi is a bit shrieky with a slightly metallic twinge. Success wise, Kelly has trumped Demi with more #1 albums and singles. By the time Kelly was 23/24, with just her second album, she gave us Breakaway, Since U Been Gone, Behind These Hazel Eyes and Because of You. Not quite sure what song of Demi's reached the peaks and iconicness of those songs back with Demi's second album or with an alum that she released in her 20s.
Demi's cute and all but it's time for her to stop pandering to teenbop and target her music towards the adults.