Much faster taking the bus. If you were to walk through Ecpot, it would take you about 20 or so minutes to get from BC to the front of the park...and it could be longer if there are a ton of people in there. In fact, I think my best time, from the front of Epcot to the Intnl Gateway, was about 22 mins, but it was early evening and there were a lot of pople milling around.
Then, you'll have to wait for the monorail to come, that could take up to 10 or so minutes, then the ride to the TTC, another 15 mins, then change over to the MK monorail line and over to MK, another 20 mins. That's close to an hour.
If you take the bus, it will be much less time....about 40 mins, and that's figured with a wait for the bus. 10 min wait for the bus, 10 min on the bus if you have to make other stops, 20 mins to MK...so is would most likely be even less than 40 mins.