It costs the same (as it should!). If you feel you will travel a lot, get the bigger book. If you feel you will REALLY travel a lot and will require even more than 52 pages in a couple of years, include a cover letter asking for additional pages. A friend of mine got a new passport with additional pages, but in his case, he included his old passport which had over 100 pages after additions. They gave him a new passport with 2 extra sets of pages (so about a 100? I'm not sure).
I got the card, and I've really no use for it other than extra evidence of US citizenship. I take it around with me in Germany and they sometimes accept it as proof of ID.