I can comment a little bit on this topic as well.
We have 2 female dogs. Both are only 2 months apart in age. Under the guidance of many large breed dog owners on a website, I waited until my dogs were 18 months old to have them spayed.
My now 3 year old Saint Bernard has mild hip dysplasia. I don't know if it would be worse or better if she were spayed earlier, but I just did what I had read and heard was best. So later-spaying does not eliminate all risk of HD, as evidenced in my Saint's case. But if I had the choice over again, and I thought it may help prevent HD, I'd still wait and spay her at an older age.
The costs of neutering/spaying a nearly full grown dog that can be 100+ pounds will be significantly higher than that of the same procedure in a 6 month old/50 pound pup, I will tell you that. So take that into account as well. Some areas have extraordinaril
y high vet fees for neutering/spaying large dogs.
I did not notice a difference in either dogs' personality after the spays were done. I do know that it is hard to keep 2 energetic, playful, big 18 month old dogs from not playing too hard or running and ripping out stitches though!