Will you be renting a car? The Disney Club gives a good discount at National. You can get up to 20% off the rental plus use a coupon for a free day on the weekend. You can also join their Emerald Club for free with DC membership. The discounts are good for all rentals, not just those in Orlando. The car rental savings more than paid for my membership.
For example, I was able to reserve a full size for 8 days for $238.18 and that includes all taxes and fees. It's for our trip this coming December. I also could have gotten a compact for $160, including all taxes and fees. Car prices seem to go up and down, though, so I'm not sure if those rates are still in effect.
That said, I still check other agencies because sometimes they are less expensive than National, even with the DC discount. I usually start my research with a visit to MouseSavers.
Enjoy your trip!