Hi Debbie!
PhotoPass has so many great services (which I love) both online and in the parks. The Photo Centers throughout the Disney parks and resorts are a convenient resource for guests like you and I that enjoy capturing those magical moments. That being said, I do feel that sometimes the price of convenience is a bit hefty. If you need to download pictures various times during your trip, then the cost of doing so may add up pretty quickly, and I would much rather spend that money on a special experience at Walt Disney World! You might be able to get away with a regular CD of your photo downloads, which costs about $12 (subject to change), but I'm inclined to think that you'll need the DVD, which is currently closer to $17. I'm going to assume that you won't be traveling with a laptop to download your photos onto each day, either.
If you are confident that you'll reach the capacity on your memory card, then my official recommendation is: consider purchasing new memory cards. Having done a search online to see the current prices, I think the investment would be reasonable compared to the cost of downloading onto a CD/DVD each time you need to. Even a proprietary memory card, such as the kind I need for my camera, appears well priced. In addition, it's easy to travel with extra memory cards and you won't lose about 20 minutes each time you would need to download onto a CD or DVD at a Photo Center while on your vacation.
Thanks for your great question! I hope this helps you in preparing to capture those extra special memories. Let me know if I can help you with any more of your planning!