Firewire vs. USB


I'm this close to purchasing a new quad core iMac. My only hesitation is that the iMac has only a single Firewire input. At the moment I use a MacBook 2 GHz with an Edirol UA-25 USB interface and two external Firewire drives, one for recording audio and the other for samples.

But what if I need to replace my interface? There are far more Firewire audio interfaces than there are USB offerings. I assume that daisy-chaining the two external drives plus a Firewire interface might over-tax the Firewire input. On the other hand, the most I ever record at one time are two tracks. I guess what I'm really asking is this: Given my need for only two inputs, what's the difference between a USB and a Firewire interface?