Ugly people can be fashionable, so since this forum is about fashion sense and not about pure looks, I will compare them based on their styles.
William: He is very preppy. He wears clothes are are simple, understated, timeless. He doesn't experiment with fashion. He wears what he knows about, what is comfortable for him. That is just his personal style anyway, which translates naturally into his fashion preferences.
Andrea: He's a bit more cool, hip, trendy, however you will say it. He likes to wear his skull-and-bones things or his sharks-teeth necklace or puka beads. He is more like a rock star than a traditional prince.
William has that boring, but tested Etonian style. Andrea is more "art-school" than Eton. I really like both of them and their styles because neither one is being anything but themselves. They don't "try" when it comes to fashion. They just do what they like. But I have to like Andrea slightly more for two reasons that have nothing to do with fashion or looks:
1) Andrea and I share the same birthday (call it loyalty to a fellow Gemini)
2) Andrea is a Casiraghi and I have a weakness for Casiraghis.