It really is a 50/50 split for me, here's my thoughts
1. DS16 is a wrestler, and the
free dining plan while at GF may kill him. It takes a lot of will power not to overeat while at buffets and sit down restaurants. The room only discount for YC CL would give us a light breakfast, counter service lunch (maybe even a split meal), evening apps, and maybe another CS supper or snack would work better for him. I feel bad taking him to WDW during the wrestling season.
It's not about the wrestling, doesn't he have a prom coming up in the Spring? 
2. DH and I had supper at GF, and didn't think it had that warm Disney feeling. I get the impression that will be quite different at Christmas. This will likely be our only Christmas trip to WDW, and the GF looks amazing.
It will be awesome, and please keep in mind you don't have to eat there. Grand Floridian Cafe is generally pretty good though.
3. Storm Along Bay looks perfect for my teens to enjoy while DD3 naps. We're cold blooded Minnesotans so as long as it's nearly 70 degrees, we're in the water.
I hear ya, but what if it's not 70?
4. The YC CL would also be a great place for teens and a parent to relax while DD3 naps.
Absolutely! No question about it.
Thank you to everyone for your input. You've switched my thinking to GF. There is always something to do at MK, and a quick trip to the park for a few hours here and there may fill our touring style.