First let's start with the history of the technology... Firewire came first in 1995 developed by Apple Inc. It was designed for high speed real-time data transfer and replaced firewire and scsi since it was lower cost to implement and had a more flexible cable system. USB was then created in 1996 as a new standard that was meant to replace serial,parallel,ps2, and many other port types. Part of the core tech was the fact that the user is able to connect/disconnect devices without powering down or restarting the machine (with proper eject on the software end).
Both technologies have evolved over time and have gotten better. Firewire initially launched with the 400 spec that has the speed range of 100, 200, or 400 mbit/s and later on the 800 spec was released with the speed range of 786.432 mbit/s with higher speeds on the horizon. Usb launched with the 1.0 spec that had the transfer speed of 12 mbit/s, later on the 1.1 spec got with a 480mbit/s a vast improvement over the earlier spec... the 3.0 spec promises higher speeds with its release later on.
Now for the major advantages that favor firewire over usb.. Usb is host mode and relies on the host computers cpu and tends to suffer from overhead and Firewire is peer to peer based thus the controller on the drive, camera, etc talks back and forth with the device controller on the computer allowing for faster data transfer.
Now I have had the chance to personally use firewire when I got my WD My Passport Studio drive for my MacBook... it is capable of Usb 2.0, Firewire 400/800 and wow the transfer speed is majorly faster than usb2.
In the end always go for firewire whenever possible if you got the choice since you will have higher transfer speeds in most situations.
Enough of my ramblings... time for me to stop writing and post this :)
Sources
Firewire - http://en.wikipedia.org/wiki/Firewire
Usb - http://en.wikipedia.org/wiki/Usb
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