I used to be a dropbox fan (actually, I guess I still am), but over the past year I gradually became more reliant on sugarsync. I actually have paid accounts with both dropbox and sugarsync. I was using both, but I ultimately realized that sugarsync was doing everything that dropbox was, but sugarsync made it easier. Eventually I bought a new Macbook air a few months back and didn't even consider installing dropbox on it since I was fully reliant on sugarsync by then.
Pros of dropbox: Like everyone knows, it just works. I never had any significant technical problems with it. It is somewhat of a pain being forced to either keep everything in your dropbox folder, or go through the hassle of creating symlinks as a work around, which did bother me everytime I needed to setup another sync relationship with another computer.
Pros of sugarsync: When I first tried sugarsync a year ago, I did have some technical issues with it being slow. However, an update about 6-8 months ago seems to have resolved all issues that I had, because after that, it has been as fast and stable as dropbox. Absolutely no problems. I find that it offers all the benefits of dropbox, plus some additional features that dropbox doesn't. The biggest for me is that you aren't forced to keep everything in the dropbox folder or create symlinks. You just select the folders you want synced to the cloud, and which folders you want synced between computers, regardless of where they are. At the time I purchased a sugarsync account, they also offered more storage for the money than dropbox. However, I don't know if that is still the case today.
Both are great products, but I found sugarsync to provide me with more bang for my buck, so to speak.