I think about getting a SAN device since this Minis fastest I/O port is Ethernet.
An early Mac Mini has the following ports
FireWire (IEEE 1394) 400 Mbit/s max 10/100/1000 Ethernet 1000 Mbit/s max USB 2.0 480 Mbit/s max
I'm not sure the difference in speed should be the primary purchasing criterion.
Perhaps, for the cost of a SAN device, you can buy a NAS device that provides better IO.
How good is the performance of iSCSI SAN devices?
http://www.rainiersolutions.com/RainierLibrary/iSCSI%20SAN%20Performance.pdf