I think a PF Willow should have two heads, just like Spike. It seems to me that a Season 5 "Tough Love" Willow would be a good choice. In that episode, Willow uses dark magics in an attempt to defeat Glory. You could choose to display regular, friendly Willow or wrathful witch Willow with the black eyes and focused/angry expression.
One of Willow's defining traits is that she is a witch, so it wouldn't make much sense to make a vampire Willow figure instead of a witch Willow figure. Also, in Seasons 7 and 8, when Willow uses her magic, she again has black eyes, but regular-colored hair. So it seems to me that simply having black eyes is the standard witch look. In my opinion, it would be better to have a generic witch look than a dark witch or white witch look.
I probably wouldn't buy a PF Willow PF though because I would want a PF Xander to go alongside her, and I'm pretty sure that a PF Xander is out of the question.