Depending on time of year, they could all share the same buses, so transportation is not a very big factor. As far as who is closest to which park, the BC is the closest walk to Epcot, about 4 minutes. The YC is only another 2 or 3 minutes and the BWI comes in at about 10 minutes. As for MGM, BWI is 10 mintues, and the BC/YC come in at about 20 minutes walking time. They all have resort boat launches which take an equal amount of time, depending on which direction you're going.
At the BC/YC, the swimming area is a huge draw for families. For some reason, you do tend to see more adults on the YC side. As they share the same exact amenities, my guess is that the beachy theme is a little more attractive to those with kids and the yachting theme, appeals more to adults.
BWI does not have any interior restaurants, so you do need to go outside to the Boardwalk for a meal. Again, just something that might make it appeal to adults. Not to mention the nightclubs right there as well.