We got our sheltie spayed at around 6 months, and she continued being her wonderful active self, we saw no personality changes. With my border mix we waited until her first heat at 1 1/2, and I will tell you the hormones from that heat really did a number on her. She became very territorial, and aggressive towards our sheltie and our cats. We got her fixed as soon as possible after that happened, and in a few months she was back to normal, but it left her with fearful aggression with strange dogs. I will always try to spay before the first heat now.
There are concerns about spaying before the bone system and ligaments and tendons are fully grown, so you might want to check with your vet to make sure your girl is fully developed, some studies have shown a connection between early fixing and hip dysplacia and tears..
