Hi Lesley,
I see two primary differences between OKW and BWV. The first is the type of resort -- BWV is attached to a Disney luxury hotel; OKW is a stand-alone resort, which is more like a condo complex. So, BWV has the amenities of a hotel, OKW has the open space and parking at your door. BWV is in the middle of the Boardwalk action; OKW is quiet and tranquil.
The second difference is the location. BWV is located between Epcot and MGM; OKW is located near Downtown Disney. BWV is obviously within walking distance of two resorts. You will hear comments about OKW being "out of the way," but if you look at a map, you will see that it is located almost in the geographic center of WDW, and is reasonably close to all of the theme parks. (I can drive to any theme park from OKW in 10 minutes)
Despite the comments above on the buses, I think most people will tell you OKW has the best buses of any of the DVC resorts. All resorts provide free bus and/or boat transportation to each of the theme parks and DTD. However, over time, I have heard a lot more complaints about the shared buses at the Epcot resorts than I have about OKW. As mentioned above, the boats take you to SSR and DTD. From BWV, the boats take you to Epcot and MGM.
Both resorts are great choices, and both are far better than most vacation accommodations you will find. I would make my choice based on what type of vacation I was looking for. If you prefer the hotel/in-the-middle-of-the-action ambiance -- or if you are going to spend most of your time at Epcot and/or MGM -- choose BWV. If you want a little peace and quiet, choose OKW. If I were driving (which we always do), I would choose OKW for the park-at-your-door convenience.
OKW will cost slightly less and the unit sizes are larger, but I wouldn't base my decision on that. It's a negligible difference when you consider the overall cost of the vacation, so I'd choose the one which contributes the most to your overall experience.