This paper presents a comprehensive modeling framework designed to evaluate and compare the performance of high-voltage vertical Gallium Nitride (GaN) PN diodes and merged PN-Schottky (MPS) diodes. The process begins with the fabrication and packaging of the PN diode, followed by the development of its physics-based Technology Computer-Aided Design (TCAD) model. Leveraging the established PN diode model, an analogous model for the MPS diode is developed, and their performances are critically analyzed through mixed-mode simulations. In addition, behavioral models of the two devices are implemented in SaberRD to evaluate their performance in a Buck converter. The results reveal lower reverse recovery and conduction losses for the MPS diode. The framework presented in this paper can aide in the design of future high performing GaN devices.