We report an experimental comparison of three-photon-absorption resonances (N-resonances) for the $D_1$ and $D_2$ optical transitions of thermal ${}^{87}\mathrm{Rb}$ vapor. We find that the $D_2$ N-resonance has better contrast, a broader linewidth, and a more symmetric line shape than the $D_1$ N-resonance. Taken together, these factors imply superior performance for frequency standards operating on alkali $D_2$ N-resonances, in contrast with coherent population trapping resonances, for which the $D_2$ transition provides poorer frequency standard performance than the $D_1$ transition.