I never really saw Cavs as agility dogs until I went to a local event and Cavs and BTs made up the majority of smaller dogs. It was insane! I've heard they're highly intelligent and somewhat easy to train as well. As far as Shelties go, from what I've heard, you have to teach "quiet" very early or they can get barky. Lillie can honestly be the same way sometimes (but usually when prompted by Keiko). The good news regarding Shelties is that there are two people very close to me that I consider borderline experts on the breed. One already offered to teach me all the grooming and training secrets if I were to get one. Plus, they could all play together (she owns a daycare/play yard/boarding facility). Both also do agility and the same lady that owns the daycare offered to give me pointers. I love Bull Terriers, but I've heard the same thing about them being hardheaded and stubborn. I promised myself that I'll always give myself a break between Boxers and get a dog a little less stubborn.