Simple.
You need the ExpressCard for the SxS, so you want the fastest connection for your external drive. That is FW800. It can be almost as fast as SATA in practical terms (for single disks).
I presume you also want to use a bus powered disk for mobility. Unless you can plug into AC power.
Get a 2.5" sata disk and put it in
a housing like this or get a complete drive. Get a 7200rpm drive if you also want to edit or just if you want speed more than space. Larger 2.5" drives are 5400rpm and a bit slower (but fast enough for quick SxS offloading).
George/