We've stayed at both BWI and BC (many many times) and while I love both of them, but BC is my favorite. Having said that, our family of five is divided regarding favorite resorts. We just returned from a BC stay and will stay at BWI CL in January. (Although I'm still on the fence about that.)
Here's what I like about each:
Beach Club:
1) Room Decor. Just a personal preference, but I prefer the theme and the decor.
2) Lobby and common areas. Just love the lobby at BC - open, high ceilings, great scent, pleasant music. The beach and the solarium are great quiet spots. Just a relaxing feel.
3) BC Marketplace. I think BC Marketplace is quite convenient, and I like that it's contained right there in the same building as the rooms - no need to go outside. The self-serve refill mug station there and at HH (currently under rehab) are really nice to have. Our room earlier this month was right up the stairs from the Marketplace and I just loved it.
4) SAB. Really no better pool on property for our family. We've stayed at BC in all different seasons (including January) and find it no more crowded than other pools - actually less so. It's hard to be cramped in a pool so large! And even if the main area is busy, the area with the small slide with a view of the lake is separate from the rest of the pool, never crowded and always fun for the kids. During warmer months, this is a deciding factor.
5) Beaches and Cream Take Out. Our boys have a tradition of late-night take-out a B&C. Sure, we could do that staying elsewhere in the Epcot resort area, but it's not as close if we're staying somewhere other than BC/YC.
Boardwalk:
1) Club Lounge. I absolutely love the club lounge here. That's the main reason our family likes it and the reason we plan to return in January. (The other reason is that while our boys will swim in January, we won't spend quite as much time at the pool as we would in warmer months.) The CMs in the lounge are so friendly and it makes coming back to the resort really pleasant. The lounge is open and airy with a high ceiling and a nice balcony.
2) Boardwalk View. Last year at BWI, we had an amazing room with a huge balcony and a boardwalk view. My husband and boys love being in the middle of the action, and that room was perfect for us. We could watch the boats come and go, see the activity on the Boardwalk and enjoy all that balcony space. I liked the fact that it's so close to the water - can't get quite as close at other resorts. And I love the view of BC/YC and the MK fireworks in the distance. It's just a fun place to stay and a great view.
For us, we'd stay either CL or non-CL at BC. But at BWI, we'd only book it CL. That's because I find the BC Marketplace and quiet common areas at BC really convenient if not staying CL. But at BWI, it's a different set-up with fewer peaceful quiet areas and no self-serve refill mug stations. (You can get refills, just not self-serve.) The pool is fine - the clown doesn't bother me as much as I thought it would - but a very different atmosphere from SAB.
Good luck with your decision - either will be a great choice!