All current Macs meet or exceed ZBrush’s system requirements. The same rules apply for PC and Mac performance as well: RAM is the most important (with 6 GB being ideal since ZBrush supports up to 4 and your OS needs RAM as well), CPU is second (with multiple cores being more helpful than faster speed), and your graphics card not mattering at all.
A newer Mac will usually have faster RAM than an older one and the processors are better as well. So you will almost certainly see better performance on the newer machine.
As a general rule, I’ve heard that a Mac will usually outperform an equivalent-specced PC.