Neither of them really "solve" sync. It's still a problem for complex applications but it works for say, 80% of the solutions.
Anyways I use both and to me they aren't really competitors. Apple doesn't allow stuff like Dropbox to have tight integration with IOS - the biggest value iCloud provides is stuff like Photo syncing, which they aren't even good at.
Funnily enough, what I do is enable iCloud photostream, then store my iPhoto library in Dropbox. The idea is that whenever I take a photo on my iPhone it gets seamlessly synced AND backed up automatically OTA.
I love Dropbox and use it on all my devices. However, I really wish they would implement permissions on shared folders. It's one of the two most requested features that users want, but they still won't implement it [https://www.dropbox.com/votebox].
As soon as someone comes along that has integrates as seamlessly on my Windows PC, Linux PC and Android phone AND offers permissions on shared folders I will jump ship.
I don't know why, but it actually makes me cross that they aren't listening to their user-base.
I know at least Egnyte (https://www.egnyte.com/) offers folder permissions, and has clients on Windows, Mac, Android, iPhone and iPad. The Linux client is available on some NAS devices and also as a VMWare appliance.
Right, no Linux desktop client at the moment. I don't think it would be especially difficult to make one, but we'd need enough demand to make the investment.
But having said that, we do provide WebDAV API. Every modern OS I know of, including Linux, can mount a file system over WebDAV. It is not very efficient, and doesn't have all the features you would get with a client, but at least you have access to your files.
Some people actually prefer to use only WebDAV, because that way as soon as you cut network connectivity the files are not accessible (security feature).
Reading these stories is a bit depressing. Coding since 5? Solving problems with Finder even Apple couldn't figure out? Is this really the level of technical accumen needed to make it in the startup world?
Anyways I use both and to me they aren't really competitors. Apple doesn't allow stuff like Dropbox to have tight integration with IOS - the biggest value iCloud provides is stuff like Photo syncing, which they aren't even good at.
Funnily enough, what I do is enable iCloud photostream, then store my iPhoto library in Dropbox. The idea is that whenever I take a photo on my iPhone it gets seamlessly synced AND backed up automatically OTA.