I have used both in the application you require, and cannot really say that one has a big advantage over the other as far as ease of installation and setup.
Give that, I would consider your ability to support either . . . count this for a third of your score. If your main experience is with the Win9X kernel, and zero experience supporting anything in the NT kernel, then XP-Pro would be the easiest to support.
Next, I would consider that XP Pro will have the greater support over a longer period of time . . . but has a higher hardware requirement. Count this for the other two thirds of the score.
I hope this helps.
Swampster