Comparison between the metallicity derived from medium-resolution gratings and PRISM spectra. We plot only objects for which the requirements described in Sect. 3.1 are satisfied for both configurations. The two distributions scatter across the equality line (in red), with a median offset of 0.01 dex and a standard deviation of 0.15 dex. In 75% of the cases the two measurements are consistent within their 1σ uncertainties.