It's hard for me to address the time issue, since we don't pay much attention to our watches on vacation. However we did both types of transportation during our July trip so here's my 2 cents.
The bus: Yes it can be tedious dealing with all the stops at all the different resorts. However you can use the time to talk to each other and to other people on the bus. We've met some of the nicest folks on Disney busses! Also, from the bus stop, the MK is just a short stroll away.
Driving: We drove over one morning because of an early tour. We had the aggravation of the roads. We ended up in the wrong lane of the parking lot - led to cast member gate....LOL! Once we found where we were supposed to park, we were close to the TTC so we walked rather than taking the parking lot tram. Then we had to take either the monorail or the ferry (we chose monorail on the way in and ferry on the way out). Then we still had the walk to the MK turnstiles. There were so many different steps in this process that the bus seemed MUCH easier and more convenient.
Of course, your mileage may vary. Have a wonderful and magical trip, whichever way you choose to get to MK!!