I don't believe the busses are shared all of the time. Perhaps during very slow times of year, or certain slow times of the day. Only time I've been on a shared bus for SWolphin and another EP Resort is after Hoop De Doo. All other times have been Swolphin only. However - one other perk about the EP resorts is that they are all pretty close to each other SO, at the end of the night, let's say there's a BW bus loading at MK, and no Swan bus in sight, just hop on. Only a few minutes walk from BW to Swan.
OP - I'd base my choice on what you'd like to be close to. If you like Hollyweird and EPcot, and you are rope droppers or late nighters, Epcot Area is a great choice. You might seriously consider hoppers, too. Epcot Area negates the travel time for two parks. If you wanna be closer to DS, and Ep and DHS are not that important, then POR is a great choice. Pretty sure POR has the boat to the springs, right?
Also, if you are a Costco member - check their travel page! Last I saw, both had pkgs.