In most of the tests I've read the firewire 400 connection is always faster in practise over USB2 on a Mac.
Limiting factor will be the HDD in the box, not the connection speed. Average HDD can't manage to fill the USB2 bus or Firewire 400 or 800...
I agree - for non random data transfers, chuck in small "random" reads an firewire will piss all over USB all day long.
The problem with USB is that it's a really poorly designed system, nice ideas - just badly executed. The problem is exaccerbated by Microsoft whos driver model is just poor beyond belief. They tried "fixing" this with the "windows driver framework" which just builds a higher level API over the top of WDM. What they should have done is chucked WDM away and gone and looked at the way linux & bsd drivers work....
Take for example the serial CDC, which for some reason (under Windows) requires an INF file before it'll work, there is no reason why this should be! OS X & Linux both have the correct behaviour and "just work".
Try transferring small amounts of data over the bus and its transfer rate becomes horrific.