So - my dog is both! Due to a whoopsie involving the breeder's young daughter letting my dog's mom in with her CKC male, Pixel is half Mini American Shepherd and half CKC.
Sounds like you already have an Aussie, so you basically know what you'd be getting if you get a Mini American Shepherd - and do make sure you get one from a breeder that CALLS them Mini American Shepherds. A lot of breeders that call them Mini Aussies are backyard breeders that aren't doing the appropriate health testing, showing, etc, and you really need to make sure a shepherd is coming from a good breeder. Poorly bred Aussies can be neurotic and aggressive. I have a coworker whose 'mini aussie' just bit him in the arm seriously enough that it left four puncture wounds. His offense was turning over in bed to face her.
Any breeder you go with - and this should be any breed - should have titled parents that are proven, be this in the show ring for show line dogs or sports if they're working line dogs. They should have the parents OFA tested and be happy to share their results with you. Don't go with a breeder who can't provide these things.
On the CKC side of things, they are great dogs! But they have health problems. Even lines from great breeders have health problems. Nearly all CKCs carrying the IVDD gene, meaning they are prone to back problems. Pixel herself inherited one copy of the gene from her father. They are also prone to heart issues, though good breeders have done work on that - Pixel's father's line has been carefully bred for heart health, and the breeder owns an 11 year old that hasn't so much as developed a murmur. But they ARE very prone to heart issues. It is something to expect going in.
Temperament-wise, Pixel is great with the animals on my farm. She has herding tendencies, but I can call her off stock no issues. She has never made a move to attack my farm animals. She will chase them sometimes, but I just call her off if it's a problem.
She is a breeze to train, and wants to work for me. However, this means she does need to have a job. If I don't give her mental stimulation, she will find something to do - and it won't necessarily be what I want her to do, haha. Honestly, she leans more towards the MAS side in regards to temperament.
If I was choosing between the two breeds, I'd choose Mini American Shepherd. They are great farm dogs and are relatively healthy. CKCs are great dogs, but they are too prone to health issues for me. I worry about Pixel developing back or heart issues from that side of her parentage.
This is my little mutt: