Xp is made simpler for the average person.
It boots 10% faster than Win2K.
It loads applications 10% faster than Win2K.
It has a different theme
some say this means a heavier video card performance the system some say it looks cool...
XP recieves critical updates to security threats Win2K does not.
If you choose Win2K you will only have to wait longer for stuff to load by a little bit. And you will use the classic theme. While some advanced performances may be harder to achieve. And lastly in time there will be many bugs on windows 2K becuase it's discontinued and microsoft no longer updates it.
If you choose XP apps will load slightly faster and boot slightly faster. You have the option of using the classic theme or the XP theme. It's simplified. The bugs are currently being fixed on a XP machine and it is supported. XP also has various media upgrades. And it offers important power managment choices for laptops.
Either way both are good but I'd choose Xp for the sake of security, Media, and power managment.