Definetely fluorocarbon. They're very used on harps. Harps are usually built to wear one type of strings or another, you can't simply interchange nylon for gut, or fluorocarbon and play straight away. So the builders always specify the strings that are used on that model. Savarez Alliance is a very used option as fluorocarbon strings. I'm pretty sure the material they use must be the same for ukulele strings.