Not sure about future travel plans? I'd say go ahead and get the book rather than the card. Wait times on Passports can be up to a month or even longer, so if opportunity strikes and you get a chance to travel internationally don't be caught without the proper documentation. An exception here might be for children or people who have absolutely no interest in flying internationally - in that case the card may save you over half the cost of a full Passport.
The following is from the US Government Passport website:
THE U.S. PASSPORT CARD IS NOW IN PRODUCTION!
We began production of the U.S. Passport Card on July 14, 2008. To date, we have issued over 700,000 U.S. Passport Cards. Applications for the U.S. Passport Card are being processed in approximately 3 weeks from the time of application.
The wallet-size passport card is convenient and less expensive than the passport book. However, the U.S. Passport Card cannot be used for international air travel. This new travel document can be used to enter the United States from Canada, Mexico, the Caribbean, and Bermuda at land border crossings or sea ports-of-entry.
Note: A current or previous passport book holder, who is eligible to use Form DS-82 , may apply for a passport card as a renewal by mail. First time applicants for a U.S. Passport, and those not eligible to use Form DS-82, must apply for a passport card in person using Form DS-11.
The card has the same period of validity as the U.S. Passport Book: 10 years for an adult and 5 years for minors under age 16. The card costs $45 for a first-time adult passport applicant and $35 for all minor applicants under age 16, regardless of whether they are previous passport book or card holders. Adults who already have a fully valid passport book may apply for the card as a passport renewal by mail and pay only $20.
