I own an early SmartDisk FlashTrax and it has been fantasic, backing up many gig of data in cars, at hotels, campsites, and overseas. They have newer better ones now. Those, or the Epsons, are a good choice.
I feel any hard drive based system WITH a display are better than the portable CD/DVD burners as you dont have to carry stacks of media, and their capacity is higher. They also act as USB drives and MP3 players for added flexibility and entertainment on long trips.
The HDD based devices without screens are more compact with just as high capacity, but seeing my shots, showing them on any TV for immediate enjoyment, and even zooming to evaluate sharpness and check histograms makes my FlashTrax a true "must-have" accessory. Get a 12-volt car charger and have fun!