Dropbox by far. iCloud is useful, and definitely a step-up from just being tethered to iTunes, but it doesn't offer any customization at all. Dropbox, on the other hand, is extremely versatile, and can be used to sync almost anything. I wish Apple would've just integrated Dropbox instead of creating their own service, because things would've been made so much easier.
Well it offers some minor synchronization. I suppose the fairest comparison would be to take Google's product suite (Calendar, Contacts, Drive, Play (apps and music), Chrome, etc) and compare their functionality on the whole to iCloud.
I don't think they should be compared as they do not offer the same type of cloud storage. One is full control whereas the other is constricted to just what they choose.