Not many PC's have 1394b (800) and its not esupported yet in Windows (or was that with Vista or SP1).
The USB wont diconnect esp if you set it for performance not quick removal (means you need use the "safely remove hardware" option to unplug though)
I have a WD My Book Prem ES 500GB and use the ESATA all the time as USB2 is far slower, it does 68MB/Sec in HD-Tune.