For MK it's usually faster to take the bus. The MK buses drop you near the MK entrance turnstiles. If you drive, however, you must park at the Ticket & Transportation Center lot, take a tram from your car to the TTC, then take the ferry or monorail over to the MK entrance.
For the other parks, driving is usually quicker if you count the time spent waiting for a bus and then riding through the OKW stops before getting on your way. The down side to driving is getting lost, dealing with traffic, and coming back to a very hot car at the end of the day. The buses are air conditioned and you can leave the driving to them. But you may have to wait for a bus, and sometimes stand when the seats are full.
There are strong opinions both ways. I'd say try the buses a couple of times. If you don't like the experience you can always take the car next time.
