My choice would be the
Backstage Magic tour. While a Grand Floridian massage is wonderful, you can get a massage anywhere. You can only get the backstage Disney details at WDW, so why not capitalize on that. The Keys to the Kingdom tour is wonderful, it only takes you behind the scenes at the Magic Kingdom, whereas the Backstage Magic tour gets you unprecedented backstage access to three of the Disney theme parks: the Magic Kingdom, Disney's Hollywood Studios, and Epcot.
I was lucky enough to take the incredible Backstage Magic tour in early 2008. You can read about some of my experience with this
HERE. I have to say that just seeing the ride construction and repair workshop was worth the price of admission. We got to see a Jungle Cruise elephant getting new skin. How cool is that! Another of my memorable experiences on this tour was getting to briefly try on a dwarf costume from the Spectromagic parade. That thing is HEAVY. Neither of these stops are on the Keys to the Kingdom tour, while much of the Keys information is part of the Magic Kingdom portion of the Backstage tour.
Other Moms Panel observations about tours can be found
HERE and
HERE.
And by the way, with 15 WDW trips under your belt, you could be almost ready for the Moms Panel yourself :-)