There won't be any advantage for GCSE.
the FX-991ES is a good calculator to have for GCSE and A Level. At GCSE you can use it to solve quadratic equations so you can check answers in the exam. Apart from that, there aren't really any other benefits compared to a cheaper Casio at GCSE.
At A Level it can be used to numerically solve summations, integrals and derivatives so again you can check your answers. And the graphical calculators will also do this.