We analyze nucleon decay modes in a no-scale supersymmetric flipped SU(5) GUT model, and contrast them with the predictions for proton decays via dimension-6 operators in a standard unflipped supersymmetric SU(5) GUT model. We find that these GUT models make very different predictions for the ratios $\Gamma(p \to \pi^0 \mu^+)/\Gamma(p \to \pi^0 e^+)$, $\Gamma(p \to \pi^+ \bar \nu)/\Gamma(p \to \pi^0 e^+)$, $\Gamma(p \to K^0 e^+)/\Gamma(p \to \pi^0 e^+)$ and $\Gamma(p \to K^0 \mu^+)/\Gamma(p \to \pi^0 \mu^+)$, and that predictions for the ratios $\Gamma(p \to \pi^0 \mu^+)/\Gamma(p \to \pi^0 e^+)$ and $\Gamma(p \to \pi^+ \bar \nu)/\Gamma(p \to \pi^0 e^+)$ also differ in variants of the flipped SU(5) model with normal- or inverse-ordered light neutrino masses. Upcoming large neutrino experiments may have interesting opportunities to explore both GUT and flavour physics in proton and neutron decays.