If WiFi is acceptable, you might wish to take a look at FolderSync. Aside from cloud services (Dropbox & Co), it can also deal with Samba (Windows shares), FTP, SFTP (ssh with Unix/Linux), and more. You can define "Folder Pairs", i.e. which folder on your Android device should be synchronized with what folder on the remote machine. Sync can be done uni- and bi-directional.
I'm using that with all my Android devices and my PC, to keep different stuff in sync. Easy to do with e.g. photos or the music collection: Add a file on one device, and it gets synced across. FolderSync even supports tasker, which gives you plenty of possibilities to deal with: Having the device synced as soon as you're at home, or only when connected to a power source, at specific times, whatever.