I've stayed at both and definitely have a preference, but they are both wonderful places to stay. If you'd prefer to be closer to Downtown Disney for the "nightlife", SSR is the place to be. If you'd rather be closer to the MK and the other MK resorts, then VWL is calling you. If you don't care what you're closer to, I suggest the "coin" flip method of resort determination. Whichever resort "loses" this time, can be your first choice next time.
Have a great trip!