Definately hands down - the bus. You may have to stop at one other resort but you avoid the TTC and are dropped off at the gates of MK.
To take the monrail you would have to first walk or take the boat to the International Gateway entrance to Epcot, walk to the front of Epcot and exit. Then take the monorial to the TTC and switch to the MK monorial taking you to MK.
We have had excellent bus service from S&D to MK! You should get a transportation guide when you check in - if not ask for one.
Have a great trip.
TJ