  Both rotation and interactions with binary companions can significantly
affect massive star evolution, altering interior and surface abundances, mass
loss rates and mechanisms, observed temperatures and luminosities, and their
ultimate core-collapse fates. The Geneva and BPASS stellar evolution codes
include detailed treatments of rotation and binary evolutionary effects,
respectively, and can illustrate the impact of these phenomena on massive stars
and stellar populations. However, a direct comparison of these two widely-used
codes is vital if we hope to use their predictions for interpreting
observations. In particular, rotating and binary models will predict different
young stellar populations, impacting the outputs of stellar population
synthesis (SPS) and the resulting interpretation of large massive star samples
based on commonly-used tools such as star count ratios. Here we compare the
Geneva and BPASS evolutionary models, using an interpolated SPS scheme
introduced in our previous work and a novel Bayesian framework to present the
first in-depth direct comparison of massive stellar populations produced from
single, rotating, and binary non-rotating evolution models. We calculate both
models' predicted values of star count ratios and compare the results to
observations of massive stars in Westerlund 1, $h + \chi$ Persei, and both
Magellanic Clouds. We also consider the limitations of both the observations
and the models, and how to quantitatively include observational completeness
limits in SPS models. We demonstrate that the methods presented here, when
combined with robust stellar evolutionary models, offer a potential means of
estimating the physical properties of massive stars in large stellar
populations.
