While I was out a few weeks ago at GDGT live, I met a few guys promoting SugarSync, a cloud backup/sync service with Dropbox in their crosshairs. I've tested them together over the past few weeks, and here are the differences I've found.
-Syncs one folder (subfolders are fine).
-Your dropbox looks identical across all your computers.
-There is a wide variety of third-party apps that write or read to Dropbox for all sorts of nifty purposes.
-5GB free, with an extra 250MB for each person you refer.
-Syncs any folder, as well as a dropbox-like "magic briefcase."
-You can merge synced folders across computers: for example, my desktops are all synced so a change in one pushes to the other.
-5GB free, with an extra 500MB for each person you refer.
To my eyes SugarSync comes out on top by a slight margin, but they are both fantastic services that make my life easier every day. Of course, why not use them both? They don't conflict with each other (you can even add your DropBox to SugarSync), and 10gb of free storage is enough to keep anyone's important documents. If you choose to use it for media backup, you'll probably need a paid account, and if you have >50gb of photos, videos, and the like, a larger-scale backup service like BackBlaze is probably a better bet.
Bleg/Full Disclosure: if you sign up for either service, please use my links- I get a little free space for each person that signs up through them.