I read that those cards have RFID chips in them.
I believe new passports are being made with the RFID chips, also.
I don't anticpate traveling by air outside of the country for quite awhile, so a passport card would save quite a bit of money (I have to buy for 7). And by the time I do need a passport I can switch from passport card to passport without having to do so in person. Some border states (such as Vermont) have agreed to issue enhanced driver's licenses that can be used for land travel.