HGVC is very close to Discovery Cove, more so than the Vistana. We haven't stayed there but we recently stayed at Cypress Harbour which is a bordering property and thought the HGVC looked beautiful. It also has one of the highest ratings on the TUG board (timeshare users group).
We have stayed at the Vistana and we really enjoyed it. There are, however, several older sections. I think I'd choice HGVC for the overall quality of the property and it's proximity to Discovery Cove and Sea World (which I'm sure you know you'll get 7 days free admission to with your Discovery Cove addmission).
Hmm, just re-read your post and you didn't actually say you going to DC so maybe this answer wasn't much help!! :~)
Jenn