I'm no expert, but I'm guessing the 50mm prime would be best. Because MNSSHP takes place in the afternoon/night, you will encounter many more situations where you are working with low light rather than high (I would think). With the 50mm prime you will be able to get good pictures of the indoor rides, good pictures of the decorations, and the Boo to You Parade.
For MNSSHP i would think the need for a zoom with a higer aperture would go down, unless you also wanted to bring a tripod to get pictures of the fireworks and the castle lit up.
Good luck, have fun and take lots of pictures!
Ann
If you can only bring one, I'd bring the 28-135 IS (f 3.5-5.6 right?), if just for the flexibility it offers. The IS feature will allow you to get away with a bit higher of a f stop than is ideal in low light conditions.
I wouldn't bring even a 50mm f/1.2 L lens on a WDW vacation. Too crowded to do the 'ol zoom via the feet thing.