Never hurts to put it to the test
Some stuff ive currently got hooked up to my Dell Precision 650 desktop box.
C: RAID1 - 2 x 400GB SATA - 44MB/s - 4.7% CPU - 13ms
R:Firewire - 1 x 400GB PATA - 32MB/s - 2% CPU - 17ms
I: IDE - 1 x 300GB PATA - 28MB/s - 4.9% CPU - 13ms
J: USB2 - 1 x 250GB PATA - 19MB/s - 18% CPU - 16ms
K:USB2 - 1 x 400GB PATA - 17MB/s - 13% CPU - 16ms
S:USB2 - 1 x 100GB 2.5" - 17MB/s - 17% CPU - 16ms
Disk K and R are exactly the same disk and casing, just a different connection since I only have 2 firewire ports and didnt want to daisy chain them. Guess I'll reconsider that
It's even more amazing that the firewire disk is faster than the internal IDE disk, although that one is older and heavily fragmented.
So, at least on my system, my external 3.5" PATA harddisk is much faster on firewire than on USB2. It's especially interesting how firewire puts no load on your CPU.
I'll personally still use the 2.5" laptop HDs while on the go. I aim for portability/weight, and the speed difference doesnt bother me while im relaxing on a liveaboard 
Cor