Usually, the bus is quicker (10 minute trip); just depends on how long you will wait for a bus. One thing I noticed is your plan seems a little aggressive. EE starts at 8 at Epcot and MGM opens at 9. One hour less travel time to MGM does not allow you to really do much at Epcot (enough for one attraction, at most two--other than Test Track which is a ride that lasts less than 5 minutes once you are on it, every ride/attraction at Epcot lasts 15 minutes or more once you are on it and some go to 30 minutes). To give an example: suppose you want to do Honey I Shrunk the Audience at Epcot; from the entrance you have a 8 to 10 minute walk just to get to it; then its pre-show and show last a total of 30 minutes; then you need to walk and catch the bus; odds are you do not get to MGM for opening.