The use of nonlinear dynamic analysis provides significant uncertainties on the seismic demand, especially when recorded ground motions are used. As these uncertainties strongly depend on ground motion selection and modification (GMSM) methods, a spectrum-compatibility criterion and a method based on the minimization of the scaling factor are compared in this work. The variability of a representative engineering demand parameter (EDP), obtained by subjecting ten reinforced concrete structures to different groups of records, is investigated through a sensitivity study based on the "Tornado diagram analysis." The results show that the variability of the structural demand produced by the variation of the ground motion profile amplifies significantly with the increase in complexity and irregularity of the structures. More specifically, for regular structures, the selected GMSM criteria provide very similar variability while with the increase of irregularities, the spectrum-compatibility criterion produces a minimization of the demand uncertainty.