One benefit of culling the photos while travelling is that there are fewer to backup and fewer to upload. This can save significant amounts of time.
And just a comment about the different online storage solutions. While your photos are uploading, the Internet connection will usually be useless for general browsing or other uses. Needless to say, this can make your companions rather unhappy if they are trying to send or receive e-mail, update their Facebook page, etc. I found this particularly bad with Amazon's Cloud Drive. (That was only one of the problems with Amazon's solution.)
What was attractive about DropBox is that you can specify the bandwidth to be used for uploading. So for example, before you go to bed you might limit the bandwidth it uses to something that allows other people to get reasonable Internet performance. When you go to bed, you can bump it up to a higher figure as it is unlikely anyone else will be using it overnight.
I've just come back from 9 weeks in France and Britain. By coincidence I adopted many of Steve's suggestions.
I took a laptop, a MacBook Air 11". For the purposes of culling photos, the 11" screen is fine, and the light weight and small size of the MBA makes it less of a pain than bigger devices.
I also took an external HD for my secondary backup. I also did not format the camera cards until I next wanted to use them in the camera. So in a sense, I had a triple backup for many of the photos.
But also I signed up for DropBox Pro which gives me 100GB of storage for $9.95 per month. This did depend on wifi being available, which was not always a given even in Britain. Even when it was available, it was sometimes problematical. But a good connection would usually allow the backlog of files to be uploaded over a night.
(One thing to be aware of is that in some places, e.g., Australia, ISPs have limits on uploads. So uploading your photos may use up the month's quota in one night.)