Kelly is obviously a better vocalist than Christina, when we're talking about technical aspects of vocals. Kelly's vocals are almost always technically impressive while Christina doesn't use her voice correctly at all. I've yet to hear a well done belt by Christina.
That is not to say Kelly doesn't have her own problems. What a great vocalist should do is to give life to their music with a gifted voice, and it is what Kelly is lacking as a vocalist. (Her material being already weak doesn't help either.) I don't know why she does a better job at covers than her own record.
Christina does far better at supporting and enriching her material with vocals.
