I think it depends on who you are traveling with. Are you going to be doing a lot of character photos? Are you going to take a lot of family photos in front of the screens before dinners? Lately, it's just been me and my DH and sometimes our adult DD. Lately I have been purchasing the photobooks because we can get a lot of photos in it. I also get the 10 photo digital package to download so I have photos to print or use on Christmas cards.
On our last cruise, I honestly didn't have enough good photos to fill my photobook. We didn't wait in line for characters. We didn't do that many formal photos and we didn't roam the decks enough to get photos on deck. We were hoping for more photos on the pier, but the photographers never seemed to be ready when we got off the ship. While we took advantage of photo opportunities where we could, we just didn't get that many photos.
One thing to remember is that you can always change your photo package on board if you are finding you are getting more or less photos taken than you thought