It depends on where you are going and how you get there.
"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."
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08
We were on the VOS a couple weeks ago and we decided to carry both versions, but in different places. I carried my Passport Card in my wallet and my wife carried her Card in her purse. I carried both of our regular Passports in a carry-on case with all my other papers for check-in as I have done for past cruises.
Carrying both versions of your Passport, in different places, greatly decreases the chances that you will be missing your Passport at check-in or later during disembarkation.