Spoisal,
Don't go via Epcot unless you have an Epcot pass. The bus station is one side of epcot, the exit to Boardwalk (International Gateway between UK and France) is on the far side. Also, although Boardwalk is only a short walk from International Gateway, you have to add in the walk all across epcot, making it a long, long walk from the bus station.
MGM is a much better bet. You don't have to cross the park so don't need entry tickets. The MGM bus station is on the way to the Boardwalk path, so when you get off your bus, you are already part way towards the Boardwalk. When you get of the bus, just walk AWAY from the MGM entrance, along the canal. Keep the canal on your left hand side as you walk to Boardwalk.
Although Boardwalk is closer to the Epcot Park, it is much closer to the MGM bus station.
Andrew