Just read the first 5 pages of Kelly vs Carrie. Total mess
Anyways, Adele. Christina has the better range, no questions asked. She lacks control though. A lot of the times when she sings, she screams all over the place and she hardly ever sings just a ballad without doing some huge notes in the middle randomly that don't belong.
Christina has an amazing voice, but I think Adele has this one won.