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.

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.

About the usb dac thing, and I know that isn't what the original poster was asking - while Firewire Dacs are few, firewire to spdif converters aren't, and they are relatively inexpensive ($200), so you wouldn't be limited to a USB dac - you can get any dac you wish.

I suggest you read the set of ten interviews on Positive Feedback that I linked to in my original post. Pay particular attention to the interface most of the designer / manufacturer / engineers are using and why they prefer it. I believe it is the first question each of them ask. It's interesting reading. Most, but not all, choose USB for very similar reasons.

In practice I would completely agree with you regarding USB not actually running up against any walls when streaming audio. I was not, however, talking about robbing the processor of any speed or capacity, it is robbing the USB bus of throughput/device. A USB 1.1 bus has a maximum speed of 12mbps and USB 2.0 480mbps. The more devices that are using the same USB bus, the more limited the actual transfer rate of each device becomes. As I said, I've never run up against the wall except when using the computer to do other memory intensive processing at the same time and I don't think that had to do with the bus being taxed in those cases. My point was, if given a choice, why tax the transfer rate of the interface you are using to deliver the music to the DAC (assuming you are using USB which is very likely). If you use a firewire drive you will not tax the USB bus.

Assuming that they are on the same bus, does the printer USB "tax" the DAC USB only when I use the printer or is it a conitnuous thing?

The printer will "tax" the USB bus only when it is active. I doubt very much whether the throughput would interfere with the flow of music though. You can test it out yourself.