Aussies are nice dogs when they mellow out at about 2-3 years old, but you basically can't trust them a whit till then. I've trained a couple aussies and I would never trust them like I trust my Sheltie. Keep in mind, if you take out height, shelties and BCs are TOP dogs when it comes to the Agility course. I'd go with a sheltie. In fact I make a point of poking at my Agility instructor who has BCs every time my sheltie beats the BCs in agility class and practice. I think I might have to lighten up with her a bit because she recently gave me a dirty look.