I love macs, I love OSX Server - but it has few real benefits over W2K8 on a decent manufacturer's (HP/Dell/IBM) mid-line x86 server in terms of speed, reliability or ease of setup/use for a small setup like yours.
Basically OSX Server benefits from only running on known hardware, so it doesn't need drivers or tweaks to make it run fast and reliable straight out of the box. Also most configuration jobs can be done through the GUI without having to wrestle with complex command lines. It's also licence-free, one server can handle hundreds of users for a set cost.
But as I started, W2K8 on a HP DL380 G6 or similar IBM/Dell will be almost, in not just as easy, to setup, manage etc. Of course you'll end up buying client licences but then OSX Server isn't the cheapest to start with.
It all depends on who's going to support this box, what their skills are like, if they have other jobs to do and the budget available. They're all pretty much functionally the same.
edit - by the way I only just looked at that machine you linked to, their site is full of inappropriate/out-of-date superlatives - that machine is neither particularly quick nor special when compared to lots of manufacturer's kit - it looks like an old'ish machine that they've just not updated or updated their site to reflect - thought you should know why it's so cheap.