Have several Fitflops, sandals and boots, and didn't buy them for the reason moondog suggests -I agree that believing they 'shape' you does show naive credulity - but because they are very comfortable. In my experience, more comfortable than competitors, whether cheaper or not.
Have considered both those styles myself, though in the end didn't get either. Do quite like the Floretta, but think their Fiorella is prettier if you want a 'flowery' one, and just didn't particularly have a use for the Frou, though I do think they're fun. So far this season, have bought the FF Limited Edition Manyano beaded sandals, and the espadrille-like Lolla.