Of course XP is more popular than Win2K, as most major PC vendors are induced to sell only with current versions of the OS - i.e XP - by guess who? XP does have a few features not available in Win2k as mentioned, but it also has a lot of floss that isn't really needed, in the same way that ME was just 98 burdened with extra stuff, and worse for it. More code equals more to go wrong, and more potential exploits to be exploited. Win2K was not ever very popular with home users, because pre-XP most home users where still content to use products from the 98/98SE/ME series.

Win2K - provided that you never install IE6 or later versions of WMP - will give you a lot less security traps than XP. But you will not benefit from security fixes from Micro$oft for much longer. You can also get much of the performance back from XP by disabling all the XP-specific ui flotsam, and using the old interface. But you will have to deal with the activation code of XP, and the DRM (digital rights management - as in, managing the digital rights of others on your computer) built into XP (and later versions of WMP for all versions of Windows).

So like most things, it really boils down to personal preference, and what's important to you in the trade off of features in the OS versus control over your system from a user rights perspective. You'll be able to make either work more than well enough to run Poser - well, as well as any copy of Windows can be with its "attractiveness" to viruses, trojans, spyware, et al.