USB2 has the huge advantage that almost all machines these days have those connectors, and the software drivers for using an external drive on a USB port are included in most operating systems. However, eSATA and Firewire400 (aka IEEE 1394a) are faster interfaces. So if you have either of those available on the machine you'll use a lot, that is preferable from a speed standpoint. Firewire 800 (aka IEEE 1394b) is the fastest of the bunch, but much less widely supported. However, if you wanted to put a 1394b interface card into you own computer on which you do most of the editing work and find an external drive that runs that way, that will be your fastest interface system. Then you just have to put up with slower performance when you take it to another machine that works only on USB2. Having three interfaces possible (for example, USB2, eSATA and 1394a) may be nice because it increases the possibility that the "away-from-home" machine might have a faster (than USB2) interface available.
Here's a question I don't know the answer for: if you have IEEE 1394b on your external hard drive, is that backward compatible so that you could plug it into a IEEE 1394a port on another machine?