I wasn't sure whether to post this under PM or Mattel Kirsten but here's a comparison of a PM Kirsten (~1996) and a Kirsten with a brand new (Feb 2010) Mattel head:
My Kirsten is on the left, as is probably obvious. The difference in face coloring (skin as well as lips and cheeks) is even more dramatic than I expected, the new Kirsten has darker eyes & hair about what I expected, but I was completely unprepared for how dramatic the difference in face shape is. My PM Kirsten has a flatter, perhaps squarer face over all. New Kirsten also has shorter, less insanely full bangs. These are of course just two examples; I think it's clear from other posts that there are variations all over the place, not just PM vs. Mattel. But these are a particularly strikingly different pair.
PM Kirsten is rather pale; Mattel Kirsten looks like she's wearing too much lipstick. The new bangs look more reasonable to me (never mind the discussions about Kirsten having bangs at all). There's a certain charm to each face shape.