I have been to both the Cinderellas Castle breakfast and the GF dinner.
Both are excellent and you see cinderella at both. The Cinderella breakfast is a buffet, but they bring it to you. All you can eat, and very good food too. You see Cinderella, all her sisters and the wicked stepmother - my 4yo daughter lived it, did it on her birthday last year.
The dinner at Grand Floridian is also very good, excellent food, you see cinderella, prince charming and other misc characters(they vary daya to day). Well worht ht emoney. Both fill up fast so make your PS ressie 90 days out.
As for which one I would choose? though call. Eating in Cinderella castle was a thrill, but when we ate at the GF we were able to ride up front with the driver of the monorail and that was cool too.
Do both - you won't lose. The price is not much more than you will pay for food elsewhere in Disney.