Aussie, border collie, corgi question?

Ok, so my girlfriend and I currently own a female australian shepherd. She is about 9 months old right now and she is a blast!! We have been tossing around the idea of getting another dog. We have come to the conclusion that we want another herding type dog. My question is, what would be a better match for an aussie?? A border collie or a corgi?? With the corgi we are worried about leaving a small dog with our aussie and our aussie potentially hurting the corgi accidently. With the border collie we are worried about it going insane while we are at work. Our aussie is fine when we go to work. We don't crate her or anything. Obviously, we play and train with our aussie when we get home. We are just getting her started in disc dog. Suggetions??