We've done both. Our first trip we did autographs and then we decided on our next trip to just get pictures with the characters because honesty, we never looked at those autograph books again. My girls didn't seem to care that they didn't collect autographs, but it did seem as though the girls didn't know what to say. The autograph books seem to kind of break the ice for my girls.
For our most recent trip, I made autograph books using pictures of my girls with the characters from past trips and had the characters sign these pictures. I did add some blank pages at the back for characters we had never met before. This made for some great character interactions and we have looked through these books since because they're photo albums. I'll likely make updated version each trip. The back cover of the books have info stating which trips the pictures come from as well as which trip the autographs come from (there is space for text in the template). I used shutterfly for the books.