I've stayed at the GF and YC (club level at the GF and 'normal'/club level at YC).
I prefer the YC
The GF is a beautiful resort, but I found the cast members lacking (both club level and on the front desk - not very good for the 'flag ship' resort), the lobby was 'mobbed' during the main hours of the day with people arriving for the restaurants/to look around, I wasn't overly impressed with the monorail either (during peak times the monorail was often full and didn't stop, so you had to wait for another one), although I did like the little boat to/from the MK. I wan't over impressed with my room view either, but that's another story
Now, onto the YC. Again, it's a lovely resort, I feel it's more 'classy' than the GF in an understated way. Every CM I've encountered at the YC has gone over and beyond.
The rooms are newly refurbished, and like the resort itself, quite understated. The loby is not as busy, and it's nice to take time out and relax. Of course, Stormalong Bay (the pool at the YC) is one of the best, if not the best, on property. I love the location of the YC, there's a fantastic selection of dining opportunities both within the resort, over at the Boardwalk, or at Epcot, all within walking distance. You can stroll into Epcot or HS (or take the boat) any time of the day during opening hours. If you go for a waterview room (and request lagoon view) you will have a beautiful view of Crescent Lake/the Boardwalk, also some of the rooms you can clearly see Illuminations.
The only 'downside' to the YC is there is no decent counterservice restaurant, unlike the GF that has more variety (although it's not as great as the moderate food courts).
What about a split stay? I'm doing a split stay at a MK resort (Wilderness Lodge, first time) and then the YC (one of my favourites) to take in Epcot and HS.
You've got to go with what appeals to you the most, for me, the GF was always somewhere I wanted to stay, I'm glad I did it but it's now crossed off my list and I'm not going to return.