How large are the files for each image? How many photo's do you take on an average day? Do a quick bit of maths and you'll find out how many cards you'll need. It may work out that a portable hdd makes more sense than buying a succession of sd cards, but the spinning disks are a liability and SSD's are expensive and still offer no guarantee that they won't fail. A large usb drive would probably be my suggestion for flash media, but make sure to buy a named brand from a reputable supplier - there are far too many knock-off's on ebay etc.
Personally I use my netbook, back the pics up onto there, and onto a cheaper, slower card and then burn 2 dvd's, mailing one home. Only when the dvd has been checked and the pics transferred onto my pc at home am I happy.
Keeping the pics on portable media with you is all well and good but what happens if you're bag gets drenched and the data's corrupted or, heaven forbid, someone relieves you of your possessions?
In theory, cloud storage is a fantastic idea for those on the mood. In practice, from my experience it's a no-go. If you had your own computer and were regularly staying in places with a decent wifi connection you could let it upload some each night but without that option are you really willing to spend hour upon hour sat in internet cafes with pathetic upload speeds seeing your photo's slowly, slowly, slowly but surely pop off one by one?
There's no right answer but when it comes to backing up photo's from a once in a lifetime trip, there's no such thing as overkill.