Just got back from Disney

. Last year I went to the GF spa for the 1st time, I had been to the DI 3 times and wanted to see what the GF was like. I went back to the DI this year for various reasons (we actually stayed at GF this time and I still went to DI). Hopefully this will help you in your decision.
First of all, the DI has a new spa menu and their prices have gone up, so, while not as expensive as the GF, it's getting closer. If you belong to the DVC, you get a 20% discount, now back to reasons.
The DI spa is the only DISNEY run spa, GF spa is actually run by another company, can't remember who.
I find the DI spa to be so much more relaxing, it's a much larger spa so you aren't as crowded with the other guests, the showers, changing rooms, dressing areas, waiting lounge and bathroom are spread out much farther so you don't feel crowded. When at the GF last year, there didn't seem to me much room to move.
When you check in at the DI, you pay it all up front, tip and everything, so when you're finished, that's it. I like that because when I finish at the spa, I don't want to have to think about paying, tips, etc. The only change is if you purchase product, you do that at the end. And the staff has always been wonderful so I don't mind paying the tip up front. I'm sure if you have a problem, they would refund it.
DI has more of a variety of treatmets. There are things that theGF offers that they don't, i.e. couples massage and "baths" but who wants to pay for a bath when you can get a hydrotub and massage combo.
I feel more pampered at the DI, it seems you get more personalized attention, I talked to a couple of other spa goers while there and they told me they also prefer the DI spa after having tried both.
Hope this helps, if you have any questions, please email me or post here. The treatments I had his time BTW, were the Seaweed Hydrotherapy (highlyrecommend) Seaweed wrap (wonderful) and a reflexology treatment (this is one thing theGF does do better).