Taryn,
Both of these spas are run by Niki Bryan Spas. You can find out more about them at
http://relaxedyet.com, including getting some price estimates. The last time I asked about a couples massage at the Grand Floridian, it ran just short of $300, tax and tip not included. I assume that it would be much the same at Saratoga Springs.
I typically go to the Grand Floridian Spa due to location, but most of the spa-enthusiasts that I talk to actually prefer the Saratoga Springs Spa. To make it a bit more confusing, I will say that my favorite spa of all is the Mandara Spa at the Dolphin. I love the exotic, bali-nese themed setting, which is very relaxing.
But to pick, I would tell you that it is more about location at this point. Whichever of the three has the easiest transportation to and from your resort makes for the best choice. That way you are not getting stressed pre- or post-experience worrying about the transportation.
If you have any follow-up questions, please feel free to ask!
Have a magical vacation!