Well you will be traveling the same distance so route time relatively the same.
If you drive you don't have to wait for a bus to pick you up which can save 1-20 minutes.
BUT you get dropped off at TTC and will then have to park, take a tram, board either the boat or monorail then arrive to MK. With the bus you get let out relatively at the entrance to MK.
I have always preferred the busses. It is the only time on the trip we just sit and relax! It is a matter of preference but neither can guarantee an arrival earlier than the other. You drive there but still have to wait for a tram and then wait for the boat/monorail so not really saving anything.
Either choice is fine because in the end you will be at the Kingdom
The bus ride is 10-15 minutes to MK depending on traffic (Yes, I did time it). Add to that the time to wait for the bus at AOA. We never waited more than a few minutes.
We found the bus transportation at AOA to be fabulous this past May. We had our own car, but left it parked at the resort all week.