Pixologic, while having a good product today on the Mac, has had a very spotty reputation when it comes to supporting the Mac. If your main application is going to be Zbrush, caveat emptor. Zbrush 3 was more than year after the PC release, and even then, it took several months more until they resolved serious issues around Displacement Map/Normal Map generation.
In terms of other 3D applications, your choices will be more limited, and if they use OpenGL as their display technology, your performance will suffer relative to PCs. There will also be fewer customers on Mac of most other software vendors, so support will likely be later than PC customers (for example, Newtek.)
In terms of 64-bit support, most vendors are further behind in support 64-bit on Mac, than PC, due to Apple’s shifting policy regarding key OS libraries, so 64-bit applications may be further out on Mac than PC. Maxon is a notable exception here.
I have used every version of OSX up to 10.5.5, as well as Windows XP, Vista, and Vista64. While I give a slight edge to Mac, vis-a-vis the OS, the limitation on applications, the poor performance of many mac “ports” (most vendors don’t develop on Macs), and the lack of vendor support for the Mac is a costly drawback. Add to that that Mac systems at the high end tend to run 30-50% more expensive for equivalent performance, the economics of Mac systems don’t seem to make sense anymore.
If you’re going to drop a lot of money on a new system, two factors are coming up fast (e.g. the next 3-6 months): Apple Snow Leopard (OSX 10.6) and Windows 7. You might want to see what happens with these.
-K