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.
Autocad is still the most used drafting software and it, by all accounts, is much better on a PC. … but, lots of graphics programs are supposed to be great on MACS. You should see what you school is doing and what is the best match between you and your school. Right now you need it for school and should maximize what you can do in school with it. Computers are relatively cheap (when you are not in school) and most people in business replace them about every three years for one reason or another. Get what is best for your present situation.