My money is on Katherine, not because of her age, but because of her tragetory and personality. Ok, Nadia spent 500 hundred years looking for her mother, but Katherine spent 500 hundred years running and fighting. Nadia wasn't scared at all, she had just to keep looking. Katherine had to be super careful and I believe she has developed surving techniches that would make she win against Nadia.
Altought they hate each other, the reasons are quite different. Rebekah hates Klaus because of everything he has done to her, while Klaus hates his sister because she simply got sick of it and tried to get rid of him once and for all.