The cloud sync landscape may be quite competitive, but there are three big players at this point, albeit two of them are relatively new. Dropbox is the one to beat of course, it has more than 50 million users.
There are plenty of differences between the three services but, when it comes to it, all three services offer pretty much the same thing.
One big difference and perhaps the reason why you chose one over the other is the platforms they support.
Dropbox is the only one to offer a Linux client. SkyDrive doesn't have an Android app, but it's the only one with a Windows Phone app. An iOS Google Drive app is coming but is not available yet.
Google Drive: Windows, OS X, Android, iOS (coming soon), web
Dropbox: Windows, OS X, Linux, Android, iOS, web
SkyDrive: Windows, OS X, iOS, Windows Phone, web.