As far as I know, DropBox is the only service that does delta block uploads correctly and switches to LAN transfer when two synced computers are in the same network.
I know it's CS 101, but neither Google Drive, iCloud, or OneDrive do this.
Not to mention the other services have bizarre naming limits (e.g., dotfiles are forbidden on OneDrive).
Syncthing does all of these, but doesn't have the "always available" feature where the service provider keeps a copy of everything for you. If that's not crucial, maybe Syncthing is a viable alternative.
"switches to LAN transfer when two synced computers are in the same network"
Dropbox LAN Sync still requires the local network to have Internet access, because it can't sync anything locally without a connection to the master Dropbox server, i.e. if your office goes offline, LAN Sync will not work.
