MHDT Havana vs Apogee Mini DAC ?

I don't recommend against FireWire, but I think using it as your main reason for choosing a DAC is too limiting. I've used the Apogee and it's good.

I'm wondering if there is a significant reason why you would not simply use the optical digital output of the Mac? I have been using that and it works fine.

Firewire is certainly preferable to USB (long conversation), but probably not preferable to optical digital when it comes to sound. Firewire channel properties (e.g. high data transfer rate, daisy-chain configuration) are better suited to external hard drives, cameras, etc. Remember that the optical output is not performing a conversion, that is what your DAC is going to do; it will also be electrically buffered from your computer, which may be quite a blessing in case of an electrical snafu (which, sadly, computers are prone to do from time to time). Last but hardly least, whereas firewire cables are limited to about 10 feet in length (you can push this limit, but it starts to have reliability issues), optical cables can be quite long (e.g. I'm using a 50-foot cable without issues). This allows you to place noisy computer equipment far away from the listening area.

If there are good reasons to avoid optical out and use firewire instead, I'd love to hear about them.

Again, I agree with Trebejo. I find that optical sounds best most the time. I realize this goes against what a lot of "audiophiles" think, but when using a Mac, optical works extremely well.

Interesting topic for me as I am expecting the arrival of a PS Audio DLIII today. My intention is to use the optical out from Mac Mini. I looked hard at the two dacs mentioned in the title but went with the PS Audio for price and ease of buying. Actually, if a Havana had been available at used price during my search I might have gone that way.
Soon I'll begin loading cds to the external hard drive. Not sure why but there is a definite lack of motivation to begin this task.