I'm new to decorating but from my classes, i've learnt than rolled fondant is to mainly cover cakes. Like those smooth surfaces you see on wedding cakes. As for gumpaste, it's usually used for moulding roses, other flowers, decorations etc because it dries really hard. I hear you can use fondant for flowers/decorations too, but where I'm from, very humid, the fondant would be too soft and then the flowers etc would droop. Gumpaste dries solid hard whereas rolled fondant keeps it's shape but you will be able to cut through...like on the wedding cakes or christmas cakes.
hope that helps.