Jon,
I use both types of computers, too, and here is my suggestion:
Buy a Mac now or if you can wait a few months, I’d would wait and by one of the new Apple systems with Intel chips.
Both systems are speedy, and there isn’t going to be a huge difference there.
But here’s the reasons why I would get a Mac:
1. Adaptability. You can run both Avid and Final Cut Pro on your Mac. You may want to do some projects that come to you via FCP, and this way you can do both. Or you may have clients that want to use FCP.
2. Avid is a fine system, but FCP will eventually overtake it for one simple reason: when you get FCP, you buy one program and you get everything. YOu don’t have to choose among 6 different versions! Whether you’re going to edit film, HD, MiniDV, whatever – FCP is the same interface and exact same software. You never have to pay more just to work on a different kind of project. Also, Macs can use out-of-the-box hardware a lot better.
Now, I could be wrong here, but that’s the trend now. The momentum is with FCP. And if you get an Apple, then you’re covered. You can use Avid and then switch FCP if that’s the way things go. But if you get a PC, you’re stuck with Avid for a few years.
I personally don’t like being stuck…
Also, FCP comes with Motion, LiveType, etc. And FCP usually requires much less rendering when you’re working with it, so it’s a little faster (this is TOTALLY dependent on the kind of prgoram you’re cutting, though).