Core audio drivers in a mac are better than those in a PC. There are less obstacles and processes that the music must go through before it is output to a DAC. Even with ASIO or Kernel streaming the Mac Mini still has less of an obstacle before output in most cases.
On top of that a Mac Mini power supply is engineered only for 18.5 volts whereas a PC power supply is supplying anywhere from 5-12v to multiple different devices like graphics cards, motherboard, harddrives, etc. So there are many different voltages all causing noise in the system.
In terms of arguing for which is better... I'd say go for the Mac Mini. It's more upgradeable for audio and starts off at higher performance. I have used PCs and Macs for years, started recording on a PC many years ago and now use a Mac in my audio system with an upgraded power supply. The difference between my PC (with a great sound card) and the Mac Mini was night and day in terms of smoothness and noise floor.
I am currently running a firewire device (Prism Sound Orpheus), but my old DAC was USB. Neither of these interfaces are free from jitter and distortion. When you compare firewire in audio to firewire in a harddrive they are two different subjects. Harddrives function much like ethernet. The data is sent in packets, which are buffered and checked for errors before being "OKed" by the receiving computer.
With audio we don't have that leisure because we are trying to function in real time. So you have a real time firewire signal being sent down to the components. This firewire cable has two twisted pairs of power leads, a ground lead, and two twisted pairs of signal conductors. The problem is that these power leads and ground conductors all feed noise into the signal leads, which disrupts the voltage. This makes it difficult for the receiving component to see if the signal is Voltage on or Voltage off.
That's the same reason a good power supply makes a difference on a computer. At each step of the way the audio signal is created using power from the wall. It is copied like in a copy machine at each step. Think of the power as the ink. The better the power the darker the ink of each copy. If you make a copy of a copy of a copy the ink starts to get duller unless you have better power. This is the case even with digital signals. As the ink gets duller it starts to be difficult for the DAC to tell whether the signal is on or off. It also creates timing errors in that the distance between these on or off voltages changes.
A good power supply goes for any component. It's what allows for more detail resolution and better transients. It's a shame that so many companies put lackluster supplies in their gear.
From that standpoint alone the stock power supplies in a Mac are better than a PC. On the other hand, switchmode supplies are designed for efficiency, not for low distortion for audio purposes. Designing a good power supply for my Mac Mini was one of the largest upgrades I've made in over a year.
Long story short... I'd say go for the mac. There's more software and DSP capability available for it and it starts off with a lower noise floor with more upgradeable options (solid state harddrive, better power supply, Pure Music/Amarra, etc).
