We used to have 2 kelpies, great companions. Not all yappy like those small dogs (whom I hate, some are ok though) and not aggressive like some bigger dogs. They lived to about 15-16, but we lived out in the country so they had heaps of room to run around and play. Since they've died, we've moved to the city so we don't have room for dogs like that anymore, but if you have the room, i highly recommend them. Pretty much all sheep dogs are good companions really.

You could always get a different pet to a cat or dog, I've always wanted a snake or lizard