Estimator variances of photon correlation and structure functions can be calculated exactly for gaussian-lorentzian light if the total experimental duration is large compared with the coherence time. For low photon count rates, the correlation function consistently shows the better noise performance. At high mean count rates, the structure function gives the faster estimate, particularly for the high-frequency part of the signal. This suggests the usefulness of photon structure functions for diffusion measurements on highly polydisperse systems.