Play with both and make a choice. Or do like I am doing and using them all. Dropbox is the one that I use for my sync stuff. I like the fact that it's been around a while, I've gotten plenty of free space based on referrals and I can control my upload speeds. And there are clients for Windows, Linux, Smartphones, etc. And for those doing large collections like Music, or pictures..if you use it on multiple PC's, you have the LAN sync feature which is much, much faster.
I use SkyDrive as a backup online because it offers me 25GB of space as I used it previously and took advantage of the free upgrade. I don't use this for sync, but rather copy stuff to it. As said above, with a sync, if I were to delete the source, I wouldn't want to lose the backup just a few seconds later. And even if there are undelete options or revision history, i feel safer not having to depend upon them.
I haven't really kicked the tires yet on Google Drive. I got it enabled and have the client installed, but haven't used it yet. But I will gladly take the free space and use it for something.