Heres my 2 cents. I would say it depends on how many are in your party and what age. We have 4 kids, 3 of them under 10 years of age. As such we do not move very quickly. So I would opt for the later flight. However, If I only had one kid or a couple of older ones who could carry their own bags, keep up with us, etc, I would book the earlier one. It also depends on how you plan on getting back to the airport. If you have a limo or towncar waiting for you its a lot quicker than the Disney bus.
ttfn