I am a DVC member at The Polynesian and like it more than Grand Floridian. I've never stayed at the Grand Floridian but when I go in there I feel like I'm in my grandmother's parlor. It's a very nice hotel and being $400 less, that's significant. In any case, the hotels are right next together and are on the monorail loop. You can easily get to The Polynesian even be walking and can still explore that resort at your leisure.
Fab1976 makes a good point though. Depending on how many people you are traveling with may affect how much space you get. I stayed at the Polynesian in a DVC room during a split stay in January. There was one real bed, one pull out couch and one murphy bed. One bathroom had a toilet, shower and sink, another bathroom with just a shower. I wouldn't call the room spectacular, so for the savings I'd pick GF.