Very tough call. We've stayed at both and loved them both for very different reasons. Tell us a little more about your situation. Is theme more important or location? Does your family like the outdoorsy type things or a more fancy hotel? What size room would you need? How many in your party?
Advantages of VWL is perhaps one of the strongest themed hotels in all of WDW. Magnificent lobby, perhaps my favorite
DVC. Really unique grounds and lots of outdoor activities. Dedicated boat back and forth to MK.
Disadvantages of VWL is the rooms are older and felt smaller (maybe due to the dark colors in the rooms). It's a bit of a hike from the DVC rooms to the lobby/bus stop.
Advantages of: BLT has one of the best locations available. Steps from MK and the monorail. Killer views of the MK and Bay Lake. TOWL access for Wishes fireworks. Rooms are brand spanking new and 1BR units have 2 full bathrooms.
Disadvantages: sterile theme. Hotel felt like a retirement home. Simple, unimpressive lobby.
You can't go wrong with either. Both have good places to eat, plenty to do on property. If you want the best possible location for access to MK and Epcot, BLT is it. If you want a killer theme go with VWL.