If you're just trying to decide between spending a free afternoon at DTD or at the BW, then I'd probably say DTD since there's more to see and do there.
However, if you're trying to decide where to stay (close to DTD or BW) then I'd say BW! Any of the BW hotels (Swan/Dolphin, YC/BC, BWI, BCV, BWV) are wonderful to stay at compared to the DTD resorts. When staying at a BW resort you have EASY access to not only the BW, but to Epcot and MGM and even miniature golf.
The close proximity and easy access to the two parks and to the BW itself is why the BW resorts are quickly becoming our favorites. My 2.5 yr old loves to stroll over and watch Illuminations at Epcot every evening and being so close makes it very easy to take her over there. DH and I LOVE RnRC and TOT at MGM so we can take a quick trip over there and ride those rides a few times right before closing as well. It's just really convenient staying over there.
Have a great trip!