I'm by no means an expert, but we use 400 speed on our Advantix camera. Die-hard camera experts will tell you that the faster speeds yield grainier shots, but that's really only if you enlarge them really big. 400 is great because it works well when you're moving, and you can use it in a wide variety of lighting levels (with the flash if it's dark). Slower speed films give slightly crisper pictures (with some blurring in the background), but they have to be used in areas with good lighting and little or no movement.
800 (without the flash) is necessary for fireworks or rides that don't allow flashes, but you'll be able to get enough shots to capture your vacation on film with 400.
Buy about twice as much as you think you'll need before you go. Advantix film is extremely expensive at WDW. You'd be better off buying extra and having to throw a few rolls away because they expired than buying many rolls there.