This work compares two approaches to characterize the sooting propensity of liquid fuels, namely, the Threshold Soot Index (TSI) obtained from smoke point measurements and the Isolated Droplet Soot Yield (IDSY), an original metric expressing the mass of soot collected from single droplets vaporizing under a hot atmosphere representative of conventional flames. The TSI of different fuels was experimentally obtained by means of a computer-vision method, whereas their IDSY was extracted using a free-falling droplet combustion facility. The results clearly pointed to a linear dependency of the IDSY with mass fraction, allowing to propose a mixture rule for this index for a significantly broader measuring range than that allowed by the smoke point lamp. A nonlinear correlation was found between IDSY and TSI, with relevant deviations for some compounds. These deviations could advise to consider which of the available indices (TSI, IDSY, others) would be more adequate to emulate the sooting behavior.