I’d stick with a Windows machine. You will need Windows if you want to work with ArcGIS or Rhino (the mac version of rhino isn’t ready for primetime yet, and addons like grasshopper won’t work in windows). The mac version of AutoCAD is also missing some really useful commands.
If you go with a mac, you end up paying a premium for the design, and then there’s additional hidden costs with buying a windows license to run bootcamp (rebooting into windows) or purchasing a windows license and parallels to run windows and os x side by side. That’s a couple extra hundred bucks right there to get windows running on the mac.