  We report an experimental comparison of three-photon-absorption resonances
(known as "N-resonances") for the D_1 and D_2 optical transitions of thermal
87Rb vapor. We find that the D_2 N-resonance has better contrast, a broader
linewidth, and a more symmetric lineshape 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 to coherent population
trapping (CPT) resonances for which the D_2 transition provides poorer
frequency standard performance than the D_1 transition.
