To advance the design of novel MOFs tailored for specific gas adsorption properties, our study focused on the CO2 and CH4 adsorption properties of two MOF databases: mechanically stable MOFs in the in silico ultrastable MOF database (USMOF DB) and CoRE MOF 2019. The USMOF DB differs from other in silico databases by incorporating considerations for thermal stability and stability upon solvent removal during its construction. In addition, to ensure mechanical stability, we computed the bulk moduli of MOFs in the USMOF DB and selected those demonstrating sufficient mechanical strength. We showed that the gas working capacities of these two databases showed significantly different distributions: a significantly higher fraction of MOFs in the mechanically stable subset of the USMOF DB exhibited preferentially high CH4 working capacity, while most MOFs in CoRE MOF 2019 exhibited higher CO2 working capacity. Furthermore, leveraging machine learning techniques, we trained random forest models on the gas working capacities of these databases and investigated the features that determine the adsorption properties. Interestingly, while geometric features such as pore volume and surface area predominantly determined both CO2 and CH4 gas working capacities in CoRE MOF 2019, the CO2 working capacity in the USMOF DB was influenced by both geometric features and the electronegativity of atoms near the metal centers in the secondary building units. We observed that these models had limited transferability across databases, except for the case of CH4 from USMOF to CoRE MOF 2019. The USMOF DB CH4 capacities spanned those of CoRE MOF 2019, often leading to good interpolative transferability, whereas the same was not true of CO2 due to different feature importance in the models to predict CO2 working capacity. This underscores the need for continued expansion of datasets to include novel experimental and hypothetical MOFs in order to obtain more universal design rules for tailoring MOFs for high adsorption of one gas species over another. Moreover, our investigation into the structural characteristics of MOFs provided valuable insights. While MOFs with longer edge lengths expectedly tended to prefer CH4 over CO2, topologies containing a small gravimetric pore volume and large volumetric surface area (e.g., wiv and ibd) exhibited a preference for CO2 over CH4, and vice versa (e.g., myd and ssa). By analyzing top-performing CO2 adsorbing MOFs, we observed the MOFs containing SBUs with trinuclear metal clusters centered on a µ3-oxygen demonstrated a high preference for CO2 adsorption due to the accessible open metal site of this cluster. We were able to confirm the generality of this observation that MOFs with open metal sites and high mechanical stability facilitate higher CO2 adsorption. These findings contribute to the understanding of key design principles for engineering MOFs with tailored gas adsorption properties, thereby advancing their applications in gas separation and storage.