I own two pc's vista laptop and xp desktop why switch to leopard if vista works without flaws for me?

I've really been considering buying an imac so i've been looking around the web and i found that most say that leopard is faster and more dependable but i found vista stable but not astounding performance wise. While i tested out a imac out at at BEST BUY super fast at least the internet browser. And no mac fanboys or girls and im not a gamer!