Your question will provoke the standard replies from both sides, many of them emotional not rational, and related to overall OS preference rather than SketchUp per-se! One undeniable fact is that a Windows box will usually cost less than a Mac.
I use a Mac (and prefer it), but I will admit there are some features of the Windows SketchUp UI that are better than Mac. For example:
- The Materials window on Mac is very confusing
- The docking behavior of toolbars is better on Windows
- The location behavior of inspector windows is better on Windows (they tend to resize and wander on Mac)
- WebDialogs are not modal on Mac. Sometimes this causes problems.
On the other hand, Mac GUI is multiple-document-interface, so you can have several models open within the same running instance of SketchUp. In Windows, each model must open in a separate SketchUp instance. This may or may not matter to you.