We have stayed at both Yacht and Beach several times each, both regular and concierge levels. Our last stay at Beach was May of this year. Agree with previous posters about the difference in formal "feeling" between the two, with Yacht feeling more "formal." But in our experience that goes beyond decor: we have found the CM's at Beach to be more friendly, the housekeeping to be better, and the overall atmosphere to be warmer and more relaxed. (In our last stay at Yacht, we saw the same dirty dishes in the hall for two days.) Plus, Beach has Art in the lobby as a wonderful Greeter. A CM told us once that the CM's are interchangeable between Y and B, so the seeming attitude difference between the two has puzzled us; it's almost as if when they work on the Yacht side, they're told to act a certain way, and on the Beach side, to "lighten up" or something. And finally, Beach is closer to Epcot - and depending on your room location, that can be a significant difference.
All in all, after 4 stays between the two resorts, we've become total Beach Club fans. Have a great trip!