Boardwalk Inn - No contest.
Both Boardwalk & Y/B Club are equally well located for Epcot & MGM.
The entertainment on the Boardwalk at night is great - Far better than at Y/B Club. The pool at Y/B club is better and has a great water slide.
If you are going to spend a lot of time in the pool and on the waterslide go fot Yacht/Beach.
If you are in a wheelchair & can't climb the stairs to the waterslide, If you don't want to push a wheelchair over a very steep bridge to get to great nightime entertainment, go to Boardwalk.
Andrew