Most Useful OS For High-School Science Education?

I teach at a high school program for gifted students which emphasizes math, science, and technology. Currently we have two computer labs for the students: A new programming lab (all Dell PCs running XP, MS Visual C++, Eclipse, and SolidWorks for programming and CAD) and an old general-purpose lab (all Macs running OS X 10.3, with software ranging from some legacy OS 9 science applications to MathCad). Most of our students eventually pursue graduate degrees in science and engineering, and we would like them to have experience with the tools they will find out in industry. As we look to replace the old machines, there has been a push to switch to PCs with XP so that there is only a single platform to support. There are over 5000 machines on the district's network and the IT department is very small (fewer than 10 people), so the fewer hardware and software differences between the machines, the better. Without opening a flame war as to which one is 'better,' I'd like to know what those of you in the science and engineering fields actually use more in your labs (hardware, OS, software), so that we can decide which platform to support. It will most likely have to be either XP or OS 10.6, with very restricted permissions to students and teachers, as that is the comfort level of IT and administration, but I'll push for whatever would benefit the students the most.