If our experience with the bus system the last three trips is any sign, then get the earliest bus possible. Sigh... I had two *long* phone conversations with Disney CMs after contemplating and sending feedback emails following our return. I am not someone who usually sends complaints after our trips because I try to relax and just take things as they come. However, we were late to a pre-park-opening breakfast after getting to the bus stop a full 90 minutes before our reservation and that was just one of our bus problems.
So, follow the bus advice above and either take the earliest one you can or take a cab. I usually don't complain about my Disney trips, but the bus service is something that I can't understand why it goes so badly.