I really don't think there's an audio performance difference between them. Though this is based on what I've read. I've only used GPO on Windows.
GPO works well on Windows. I suggest you stay with what you know. I, personally, would rather spend my time writing music than learing new software.
Buying another Windows Box would be cheaper, also, as this side of the industry is more standardized. Macs are the definition of Proprietary.
Macs tend to have the prestige and tradition of being used by professionals, though these days there are at least as many professional audio options available to the PC. From a technical standpoint, I don't see how Macs are superior, methinks it's just a case of them using what they learned on back when Macs were really better in that field.
Macs tend to look cooler, though. Also, Macs are not targeted near as frequently for worms and adware. Though that is simply because Windows has such a large chunk of the market.
Quite Frankly, the reason Apple's been dualing the G5s is that that's the only way they could get those PPC chips to have the same performance levels as the high-end x86 processors used by pretty much everyone else. This is likely the reason Apple is going to start using x86 processors now.
Windows has more hardware options available, but with that comes more places for it to "break." Though if you read your reviews, that shouldn't be a problem.
Of course the main drawback of Windows is that everyone that writes malicious software targets it because of it's 90% hold on the market. Of course that can be remedied be keeping up to date, running a firewall, Spyware and virus scanners (keep em up to date), and using common sense.
In terms of bang-for-the-buck, a PC would be the better option. There is competition among hardware makers in this field and it keeps the prices down.
In terms of being free of viruses and other malware, Mac would be your better choice.
On the Audio Side, I really can't see a reason why one is better.
The choice is yours. Either way you'll be on a more powerful system than what you currently have, and that'll make your GPO days a lot sweeter.