This is a good read about risks and benefits:
American College of Theriogenologists
The risks are something that I think aren't talked about enough. Anesthesia problems isn't the only complication. I went through this with Ellie but finally decided to go ahead and spay. For the females, I personally feel it is best. Healthwise for the males, I'm between it isn't worth it or I'm undecided. However, because of marking and chasing females, I would most likely neuter also.