The aussies and ACD that I have met have been very different from one another. IME (mostly training classes/dog daycare/them coming into my current work) the ACD are much more introverted, quiet and more suspicious than Aussies when around strangers. They have the smooth and silent going on, they honestly remind me of dobermans when they come into the store. More "I'll tolerate you petting me, but I'm not going to come ask you to." than most other dogs. They rarely come barreling up to you for hugs and pets, whereas the Aussies I have met are more "life of the party" type dogs. ACD also tend to have the tough as nails "cattledogness" (yes, that's a word , cause I said so
) that I haven't seen in many Aussies. You also see more DA in ACD.
I've owned one ACD when I was younger, and did everything in my power to try and take one home from daycare with me, they are without a doubt one of my favorite breeds. But you can also tell how much they AREN'T the pet for everyone by doing a search on Petfinder for them. There are SO SO SO many of them in rescue, it breaks my heart.