USB 2.0 is rated at 480 MPS
Firewire is at 400 MPS
eSATA at 3 gigs a second would be best if one of your systems supported it.
I recently purchased an external HD that has all three interfaces and my motherboard is eSATA capable, but the HDD did not come with an eSATA cable. I searched all the local retailers in my area with no luck. I finely uses the USB interface and the speed at which it worked was just fine. I moved a few gigs each of MP3/Wav, photos, and regular files from one computer to the next with not much effort at all.
Next time I make a purchase online I will order an eSATA cable, but for all practical purposes I've learned that USB works just fine and I would recommend not going to far out of your way for a faster solution. Maybe only if you were using the external HDD to daily backups of large files.