Dispersion spectroscopic sensing of trace gases, measuring the anomalous dispersion at a molecular resonance rather than absorption, has experienced increased attention in the past view years. Their advantages over absorption based spectroscopic sensing are the independence of signals from laser power and their linearity with concentration, even for optically thick samples. In this contribution, we give a comparative discussion of performance, noise and limitations of dispersion and absorption spectroscopy. We relate dispersion spectroscopy to phase-shift rangefinding, for which figures of merit are available in literature. Based on our analysis we conclude that dispersion spectroscopy cannot outperform absorption spectroscopy in most experimental situations. In some applications, where the optical power reaching the detector is unstable, dispersion spectroscopic techniques can, however, be advantageous.