It would have to be at the transport layer at a minimum (TCP/UDP), more likely at the application layer where NFS and most other protocols live. Take a look at Figure 1 here for pretty good layout of how DLNA fits within the OSI model.
But to the point, you're going through different sections of the PS3 software/hardware to write the content to the harddrive vs directly playing it back, one of which is much slower than the other for a potential variety of reasons (hdd write speed, slow sata bus, PS3 firmware limits transfer speeds, etc).
As for powerlines vs wired networking, there may be potential for powerlines in the future, but right now they get nowhere near the theoretical speeds and they have many more chances for interference. It's hard to beat a good wired network.