If the Q can support a USB tuner or a network tuner, such as the Silicon Dust ATSC tuners then you can probably replace a server PC with the Q, as the Q will have eSATA and USB 3.0.
Will the Ceton tuners get ATSC support? That would make it much easier to replace a PC.
Can the Q move recorded shows to the external drives?
As long as the Echo can support high quality DVD and BD rips (folder structure, ISO, MKV) then it will have a major advantage over the Xbox 360, as the rips will most likely have to be transcoded to a native Xbox 360 format, unless you store them as wmv.
Someone mentioned the Dune player..........awesome format support. But as far as I know it, the Dune does not support live TV. I think it can stream recorded TV? So, as long as the Echo supports the "important" formats, but not necessarily all of them, then it should have it over the Dune due to the TV support. And the Echo doing all of this through the WMC UI.