If you need in-app syncing on iOS, Dropbox -- just be aware of the security limitations. iCloud has serious issues if you're not all-Apple, and moderate issues

If you need in-app syncing on iOS, Dropbox -- just be aware of the security limitations. iCloud has serious issues if you're not all-Apple, and moderate issues if you are all-Apple. SpiderOak might be a good option, although there (like AeroFS) isn't much integration with iOS apps yet. Their security model and general thinking is probably the best of the public options. (Tahoe-LAFS is interesting too, but not really ready for prime time; it's a lot more like AeroFS in design with commercial providers added on.) I still use Google Docs/Drive to share collaboratively-edited files with some people, and iCloud for iOS device backups, so I think the answer is there isn't a single answer.