Last fall I spent 2 months visiting France and Italy. I brought with me 2 cameras, a Sony RX1 and a Sigma DP2M along with 10 16GB SDHC cards. My wife also had a PS camera and a few cards.
To back everything up, I brought along an 11" Airbook, a 1TB external WD Drive (USB 3.0), 2 64GB Thumb Drives, and I had a subscription to Dropbox. In general, I backed up the previous day's images every morning to both the Airbook and external Drive. I rarely erased or reformatted any of the camera cards, only at the end of the trip did I have to reformat one card when I ran out of empty cards. I regularly backed up any groups of images that I felt good about to the pen drives. I also up loaded images occasionally to my website and to Dropbox.
How would I save weight? Well, I had too many camera accessories, including a monopod that I only used twice. The external drive with it's cable and a neoprene case always seemed very heavy, although less than that of the Airbook. The pen drives (
here) worked well. Even though they were USB 3.0, they were a bit slower than the external drive. If I was even more concerned about the weight than I was, I would just carry more pen drives and leave the external at home. This particular pen drive is rated at 10 yr. data retention, 5 yr warranty which is much better than any of the portable externals (USB-powered) I have seen.
Another issue is how much time do you want to be spending on your backups? I certainly didn't spend all that money to travel around in Europe to spend a lot of time waiting for my backups to complete. I found backing up to my website and to Dropbox to be very slow and inconvenient and eventually gave up on that option, the internet service in many hotels in Europe (at least most I stayed in) varied from poor to bad. The thick stone/concrete walls in many hotels meant that you were lucky to be able to send an short email, let alone try to up load pictures. Often I had to sit in the lobby to send emails.
I numbered all my camera cards and when one was full I moved the tab to prevent erasing and stored the card in a special case. I think the most important consideration when traveling and backing up your images is reliability. I decided against the stand alone card-reader./backup drive units for that reason. Most had poor reviews on B&H. If you want to backup to pen drives or external hard drives you need a USB port, so that eliminated the ipad for me.
Of secondary consideration is the weight and convenience. I'm planning a trip to the Himalayas next fall and I'll chose my accessories carefully (but I will bring my Gitzo 0531 tripod) and I'll bring my airbook with pen drives instead of an external drive. For convenience, make sure everything is USB 3.0.