Both good dogs but there's the issue of herding instinct yet. I'd go with a good German Shepherd before I get an Aussie. The Aussies are one of the smartest breeds of dogs in the world I think but they're so hard wired to herd that they will start herding without being told to do so.
The problem though with German Shepherds is that the stock of GOOD dogs of that breed in the USA are just about gone. Too much breeding for what the judges of conformity shows here in the USA and not enough based on performance have given us a very watered down dog with horrible health issues. If you get a good German Shepherd you'll have to go Germany or some other teutonic nation.
There's also the possibility of a Belgian Malinois but once again you need to start looking overseas. In those countries breeding of these breeds is done by performance and not purely by cosmetics. Of course even over there you still have to be careful and buy from a breeder that has a strict breeding program designed to advance the breed not just in looks but also in physical capabilities.
All in all in the US, I still suggest a great pyrenees - make sure to get one from the working/farm/field lines and you'll be fine.
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