The SMB way of doing things is very Windows-centric, not surprisingly, whilst the NFS way is very Unix-centric. How well do the two services translate permissions and ownsership from the 520's way of thinking? One may make more sense than the other, from the 520's point of view.
In addition, SMB requires a conceptual mapping on a Linux client for much the same reason, which manifests itself as things like setting default UIDs when you mount the share. NFS doesn't have that problem, so it means you do the concept translation once: on the server. However, NFS has very different error handling: generally if the server disappears, the client normally 'hangs' until the server comes back. SMB tends to return errors instead.
Sorry if I'm not making much sense. It's rather hard to explain.
Wade.