Though it pains me to say it, in most cases usability should trump beauty (except in zombies, ugly trumps everything for zombies). That doesn’t mean ugly sites are the most usable, far from it. A site can be usable without being ugly and a site can be ugly without being usable, but if you ever have to decide between keeping a feature beautiful and making more plain but usable go for plain and usable. Users won’t remember the color of the button, but they will remember how the text on it was too small and had too little contrast from the background.