Both have pros and cons. Beach Club lobby can get crowded and noisy from Cape May Cafe. Boardwalk's lobby is less chaotic. Have you considered Yacht Club? It's my favorite Epcot area resort and would be my pick again out of the three. Beach Club doesn't have a balcony in all rooms, which is a priority for me. Our stay at Boardwalk was in December 2016 (the TR is in my signature). At the time I said it was a one and done, and was sad b/c it was a dream resort for me. It did grow on me, and I would definitely stay there again, but I do prefer Magic Kingdom resorts overall.
For two adults, I would choose Yacht Club or Boardwalk.