Which one of the twins? lol If you want resilience and a fight and someone who WON'T give in Rex would be the choice. If you want a good battle for Dominance but someone who will eventually give into demands every now and then it's Aries you want.
I say Aries just because Rex is VERY dangerous and can take on multiple people at once even supernaturals and if she's tortured she still won't give in because she's nuts lol Aries won't be able to take too much abuse even with her healing abilities
plus Aries is the baby of the pups and it'll cause an uproar