We've stayed at both and most recently MGV last week for our 2nd time.
If you are going with kids, I'd go for the MGV. There is more for them to do here (fishing, biking, sandy beach, toddler pool, pool side activities, camp fire night, movie night by the pool, an indoor play gym, etc.) This a very large resort. The new pool area is quite nice, but harder to keep sight of you're kids because of the shape of the pool and it's design.
The V.V. is more luxuriously decorated inside and smaller than MGV. Their lobby is gorgeous! This resort has a great pool with one side zero depth entry for toddlers with squirt fountains, there is a great play ground and most units where only a few minutes walk behind the buildings.
Both resorts are in a great location and only a few minutes from each other by car on the same side of I-Drive.
I've noticed that the V.V. has added more buildings.
I think you'd be happy at either place.