i know it's a tough decision trying to figure out when is the best time to spay. i did a bit of research and reading, and it seems like the more i researched and asked around, the less i knew what the best thing to do was. since there are advantages/disadvantages to whatever you decide to do. so i just made the decision to do it earlier since i loved her personality, and thought that maybe her personality might change if i wait till she is older to spay her. ( i didn't know when she would go in heat either)
i ended up basing my decision on my gut feeling, because there are so many different things to consider, and i was getting confused by all the stuff i was reading.
another thing you might consider is that if she is older, maybe her 'parts' are bigger, and it would have to be a bigger cut? (this is just my guess, not anything i read) i just compared it to humans, i feel that surgeries on little people (young kids) always seems to be less traumatic than surgeries on older, bigger people. it seems that they bounce back faster.
just my thoughts
good luck!