We have a border collie - they are similar in temperament to Aussies but a bit smaller. We have a few of each breed in our group of friends, and both breeds are very intelligent, energetic, loyal and FAST (even when running higher mileage). When tired, both breeds are known for being "velcro dogs," meaning they'll stick right to your side, lay on the couch with you, curl up in bed next to you if you let them. But when they're not worn out, they can get obsessive - spinning in circles (ours does this), jumping up and down, sometimes destroying things. Our BC won't chew furniture/walls/carpet, but I've heard of Aussies and other BCs who will. You should also consider whether you have kids, and if so, whether they're scared of dogs - both breeds will sometimes try to herd children, which can scare the smaller ones if they're not used to the dog. That said, we plan to stick to these two breeds pretty much forever - it's so great to have a dog who is smart, doesn't eat everything it sees, and doesn't bark like crazy. We dog-sat for a golden retreiver a few weeks back, and while he's certianly sweet, he is so DUMB. Barking, pulling, eating trash... it was a really eye-opening experience.