Beach Club, no question. I've stayed there on numerous occasions, and it never disappoints. Stormalong Bay is easily the best pool on property, and the only one that is strictly monitored to ensure non-guests aren't at. The convenience of entrance into EPCOT via the International Gateway is such a huge plus. It's super fun to hop over to the Boardwalk at night for a drink or snack, which you're directly across from. And the overall atmosphere is phenomenal.
That all said, Beach and Yacht are pretty comparable as they are sister resorts and literally connected to each other, offering basically all the same ammenities such as Stormalong Bay and easy access to EPCOT, so that would really be your preference based on decor. I prefer Beach due to the light colors and laid back atmosphere, whereas Yacht is a little less casual with all dark wood and theming. Either way, you can't go wrong.