PC for ZB3 without a doubt.
ZB2 and ZB3 doesn’t run natively on current Macs and the plausible “solutions” (Bootcamp/Parallels/VMware) are weak at best. (I’ve tested all three thoroughly on both a quad-core MacPro and a top-of-the-line 4GB RAM Core2Duo Macbook Pro.) Yes, you can get ZB3 to run on a Mac (sortof) with significant limitations on Memory (Usually 2GB), significant difficulties to get a tablet to work, and with several unusual behaviors that PC users don’t have to and wouldn’t put up with.
I think BUSHDA is entitled to his opinion (I happen to concur), to wit: If you want a sculpting/modeling experience that is more Mac firendly, there are other choices. He was merely pointing out one alternative.
So, the decision comes down to this: Are you commited to ZB3 as your modeling/sculpting tool of choice? If so, get a PC with as much memory and CPU horses as possible. If you would preference is the Mac platform, then you should investigate other choices, as ZB and the Mac are not the best of friends.
Some here will tell you that ZB is the only way to sculpt; that is simply not case. The analogy about swimming and biking is a little askew. May I suggest that the discussion is more about the backstroke versus the butterfly and what size and shape pool do you need? 