We investigate the glass transition behavior in polymer thin films using a model equation-of-state approach, which involves molecular parameters whose values are determined from fits to bulk information only (pressure volume temperature and surface tension data). Following an earlier proof-of-concept application to freestanding polystyrene (PS) films, here we both extend the study to poly(methyl methacrylate) (PMMA) films and generalize the model so that it is applicable for either freestanding or supported films. In the case of the freestanding PMMA film, model predictions for the T-g suppression (relative to bulk) as a function of film thickness' are in very good agreement with the corresponding experimental data, reflecting the fact that freestanding PMMA films are evidently less perturbed by the presence of free surfaces than those made of PS. We then turn to the case of PMMA films supported on a silica substrate by accounting for possible polymer substrate interactions, such that when these are switched off the same model maps smoothly back to the case of a PMMA freestanding film. We then probe the origin of the interaction required such that the model can capture the experimentally observed T-g enhancement for supported PMMA films while also accounting for the relative lack of impact on the T-g behavior of supported PS films relative to their freestanding counterparts. Finally, we make connections between related experimental and simulation studies and our own results for the differences between supported PMMA and PS films.