The rapid integration of digital platforms into urban life has given rise to the concept of platform urbanism, highlighting the co-constitutive relationship between cities and platforms. While platform urbanism has emerged as an important approach for understanding contemporary cities, the field lacks a robust comparative perspective. To address this gap, we synthesize comparative approaches from platform studies and urban studies to outline a typology of comparative lenses for examining the co-constitution of platforms and cities. We identify three key broad comparative approaches: cross-city comparison of platforms, cross-platform comparison within cities, and simultaneous comparison across both platforms and cities. For each dimension, we propose four analytical approaches: genetic, generative, connective, and integrative comparisons. This yields a 12-cell matrix of comparative strategies, each illuminating different aspects of how platforms and cities shape one another. We illustrate these approaches with examples from short-term rental platforms. The framework highlights understudied areas, particularly the need for more cross-platform comparisons within cities and comparisons across both platforms and cities simultaneously. The proposed comparative lens enables a more nuanced understanding of how diverse platforms interact with varied urban contexts to produce distinct socio-spatial outcomes. This framework lays the groundwork for future comparative research to unpack the complex dynamics of platform urbanism across contexts.