The location benefits of both are basically the same, since they are next to each other. I think it mostly depends on how much you love Disney. At the Disney owned resorts, you are surrounded by the Mouse 24/7, from the wake up calls, etc. The Swan/Dolphin is not owned by Disney and caters to the convention crowd. I didn't feel surrounded by the Disney magic there, which can either be a plus or a minus depending on your view. (My cousin hates all the mouse ears everywhere. He says he KNOWS he's in Disney, he doesn't need it in his face all the time. I, on the otherhand, want to be submerged in Disney until I'm all wrinkly and pruney.)
I've stayed at the Dolphin before kids and it was fine and have never stayed at BWI, although I've been in its lobby and walked on its grounds. THe BWI seems to have some clever pools, one has a slide that looks like a roller coaster. On the downside, you could have a noisy room if you're too close to the boardwalk, although that might just be the villas that are near the clubs.
Great room service at the Dolphin! And it's cheaper, so you probably get more bang for you buck there. You can also get "real" coffee there, and not the Nescafe crap they serve througout Disney. On the downside, I didn't care for the dark, and to my eye, wierd, theming there, but that is all a matter of taste. The Swan was smaller than the Dolphin with less resaurants there, but they seem to share all their pools etc. so I tend to think of them both as a single resort.