I would say Dropbox. It's cross-platform (Android, iOS, Windows, OS X, Linux).
If you can move files within the system
(on iOS there's "iFile" from Cydia, I don't know about Android), you should be able to sync everything together. Probably not automatically, unless you know how to run a daemon in the background (like "rsync" for Linux, which can sync multiple folders, for backup purposes mostly).
You'll also have everything "in the cloud" with Dropbox, so keep that in mind, both security-wise and wifi/3g coverage/availability. For doing this on a local network with a router... you'll probably have to create a shared network folder or something. If you plan on signing up, can you please use this URL:
http://db.tt/ZnDCBQ3 ? We both get 250 MB.