As mentioned, recent research has shown that delaying neutering in both males and females may be beneficial in some areas of health. In light of that, I'm delaying spaying on any future bitches, and may consider keeping a any males intact.
However, if you think that there is any chance that you might not be able to safely confine an in-season bitch and ensure that she won't be accidentally bred, then spaying at 6 months is the better option. Same for a male if you think you wouldn't be able to keep him from accidentally breeding someone's bitch.