firewire or USB for external music storage drive?

Firewire is probably the better choice if you are using a USB DAC. Every USB item you attach will share the bus with any others that happen to be demanding throughput at the time. So if you're running a USB DAC and a USB hard drive they will split the USB bus (and divide it further should you use any other USB devices at the same time). Using a Firewire drive will give that much more dedicated throughput to your USB DAC (or in your case your HAG converter). As Almarg implies, audio is not that demanding in terms of transfer speeds, so all of this may be a non-issue, especially if your computer is doing nothing but serving your audio signal.

USB will rob the processor of cycles, while the firewire will not.

all good except that you will find many more USB DACs then FW DACs...

all good except that you will find many more USB DACs then FW DACs

I was under the impression the poster was asking about storage devices to store the music library on, and not about DACs.

If you are asking about the DAC interface I'd be looking at asynchronous USB DACs. Firewire DACs are few and far between, partially because software needs to be written for each operating system and update thereof in order to use the DAC. There are other advantages to the USB interface from the functional standpoint. There's another thread on this subject here which gets into the interface debate where the DACs are concerned, but you'll have to sift through quite a few responses.

From that thread, definitely check out these interviews for some interesting viewpoints on the current state of computer audio.

Jax - my bad - thnx for setting me right

I think it's all been well said by the other poster -

the only thing I can contribute is that in my experience I have seen no evidence that USB robbing the processor of cycles is an issue in any way, shape or form - now if we were talking about video it might be an issue but then we'd also be talking about FW800