Two genera of terrestrial slugs (Arion and Geomalacus) display a striking disproportion in species richness in the Iberian Peninsula. While there are 17 Iberian endemic species in Arion, morphological criteria only recognize four species within Geomalacus. Sequence data were used to test whether these differences could result from: (1) cryptic diversity within Geomalacus; (2) an earlier origin for Arion (older clades are expected to accumulate more species); (3) distinct patterns of diversification rates (higher initial speciation rates in Arion), and (4) some combination of the above factors (e.g., an older clade with higher speciation rates). Species delimitation tests based on mitochondrial and nuclear data revealed eight cryptic lineages within Geomalacus that lessened the asymmetry; nevertheless, the disparity required further investigation. No meaningful differences in crown group ages of each recovered clade were found. Regardless the different premises of the two equally plausible diversification models (similar initial speciation rates vs. higher initial speciation rates in Geomalacus), both coincide on diversity-dependent diversification for the two groups but weaker rate declines in Arion best explains the observed asymmetry in species richness. Also, the broader environmental tolerance combined with a faster dispersal and wider distribution may have represented an evolutionary advantage for Arion.