We did CRT breakfast and 1900 Park Fair for dinner in April. The differences that I can recall are:
At the castle, Cinderella poses for photos (which are included in your price) in the "foyer" of the castle, but does not appear upstairs or go from table to table. She goes from table to table, greeting the children, signing autographs and posing for photos at 1900.
At 1900 you dont get (or have to pay for) the photo package.
At CRT my daughter could choose a wand or a sword and got a little yellow star "a wishing star" with a flashing light. They dont hand out souveniers at 1900.
1900 is a great dinner buffet, highlighted by a chilled strawberry soup that my daughter still talks about. CRT breakfast is pre-plated and fine, but food-wise nothing special.
While 1900 has beautiful decor, it cant compare to the magic of actually being inside Cinderella's castle. That can be very special!
So, I guess there are pros and cons for each. I would do CRT if you really want to be in the castle, and 1900 for more interaction and much better dining for your money.
Hope this is helpful and not confusing!