You can read my answer to a very similar question
HERE.
As to your specific questions:
The exact princesses appearing at any of the character meals is never guaranteed. The dining room at the Royal Table is smaller and has better sight lines so you can see all the princesses in the room at once, which adds to the ambiance, but my personal experience is that there is maybe one more princess at the Storybook meals than at the Royal Table. The one thing that is fairly certain is that Ariel in her wedding dress will only be at the Princess Storybook meals and not at the Royal Table. (You could always see mermaid Ariel at the Grotto in the Magic Kingdom.)
As far as the photo opportunities go, at CRT, the main formal portrait takes place in the downstairs lobby. Cinderella is seated on a throne and you get to stand of sit next to her. I like the whole throne room atmosphere for my pictures. The formal portrait at the Storybook meals is in front of a painted backdrop, it is probably prettier that the throne, but not as "real." Once you're in the restaurant, your photo situation will be similar, you standing next to a princess with probably some other guests in the background.
I personally would choose the Royal Table if it is available, mostly because I think the experience of dining in the castle is so magical, but the Storybook meals are also wonderful. Either would make a lovely birthday venue for a five-year-old princess.