Choose the one that sucks least
Really, all operating systems suck, and the task is choosing one that provides the least hairy-mouth experience for you users and IT staff. At the risk of stirring up the fanbois (and gurls) here is my take on it:
1) Windows XP has maturity, and best range of software and tools. Also the best range of maleware by far. On its way out, and the final death of IE6 will be a relief to all, including Microsoft..
2) Windows 7 shares most of XP, but less legacy software and hardware works with it. Needs more (i.e. modern) hardware to enjoy using it, and to get the best deal with the malware or the joke that is AV software.
3) Apple Mac solves a lot of the security issues, but less software support. And costs a lot more for hardware. Jobsian control freakery an issue long term, but most folk like it as a few key things like Office and Photoshop are available natively for it..
4) Linux has the security of Mac (if not better) and freedom (speech and beer), but not much in the way of mainstream tools work "just like that". Helps if you have a fez, and maybe a beard. Would help a lot if they could stop dicking around with the desktop and fixed known bugs - looking at you Canonical.
Training of your users is needed no matter what you do, and if you think going from XP to 7 is no problem for Joe Average (and not typical El Reg reader) you are a fool.
If you are dealing with reasonably staff, then mixing Linux or Mac for the host and running VM(s) of XP, etc, for legacy stuff works and makes security better, if a bit more involved to manage.
But don't trust my opinion, I don't have a fez.