Hello,
I would stay with the Macs. Aside from my personal loyalty to the brand, the arguement may be made that cross-platform diversity is a great idea. Let's say that a virus is brought into the lab, and it wipes out the entire Windows community. You at least have a lab of Macs there that others may use, and the internet may be used, and so forth.
Plus, you are at great risk of alienating your instructor, and she is the "customer" in this case, correct?
OS X will feel different than OS 9 to the lab, but it will work just fine... if not even better.
Now, if you price cost out the PC's, there will be in initial lower cost to get started, but the support costs... license upgrades, antivirus protection, server licenses, downtime... they will factor into the overall cost, and you should see the PC's be more expensive down the long run.
As long as you are out there thinking of things, and you mention that you are not die-hard Microsoft, you might consider a Linux server that will run Samba for your Windows people, and netatalk (or Samba too!) for your Mac people, and really cut down on licensing support costs. With VNC, you can remote manage a lot of the machines too!
Let us know if you want to go DEEP on getting this going. WOuld like to help. And if I miss on this thread, please message me to get my attention back.
Christian