I have dinner reservations for character dining at the Grand Floridian's 1900 Fair Street and also at the Castle's Round Table. We can not do both which means I need to cancel one of my reservations. I know eating in the castle would be really neat but I am really looking for the opportunity to get pictures of my daughter with Cinderella. My daughter will be celebrating her 8th birthday while we are there and we will not be visiting WDW again in the near future...so we have to make the most of our trip. Any suggestions?

Cinderella should be at both. she will most assuredly be at "her dinner" at the castle. so if it's a photo op with the little cinder girl the the castle with all it's beauty should be the place. in addition you can get a great photo with the Fairy Godmother.
so the question comes down to food choices and scenic back ground. now I have never actually ate at 1900 for dinner, but I can say that the food at the castle is pretty good.

I haven't been to GF's 1900, but I have been to The Royal Table, last was in Nov. If going for dinner you get a free picture with Cinderella as entering the castle (8x10). Once upstairs, the entertainment is the Fairy Godmother and the two Mice. The Fairy Godmother sings and the mice and her dance around. Cinderella does not appear then. The Mice do come to tables for pics. Also, I was there on my Birthday and people who had birthdays had small dome cakes with mickey confetti presented to them.

If you want the full princess experience at the Castle, do breakfast or lunch. Most of the princess line-up (and, I think, Mary Poppins, who is not a princess), are at the Castle for breakfast or lunch, and they do a little ceremony. Your child gets a little wand to keep (boys get a sword). It is expensive (2 credits on the dining plan), and the food is OK, but a little girl who is into the princesses will love it. We have done both Cindy's Castle, and the princess lunch at the Norway restaurant at Epcot, in the same trip, and it was not "too much" for my daughters. The Cinderella dinner at the Grand Floridian is fine for the food, but she's the only princess there. You will see the "supporting cast" though, including the mice. If you are only going once during a time in your daughter's childhood when she is into the princesses, do the Castle. If you want to do a sit-down meal at Epcot, try the princess meal there (we normally do it for lunch). My kids love the relaxed "face time" with their favorite characters, and it's a highlight of their trip.