The increased adoption of more electric aircrafts has been spurred by the development of onboard converters with high-efficiency and high gravimetric power density. Taking the example of a modular Regulated Transformer Rectifier Unit (RTRU) using GaN switches, this paper provides a comprehensive comparison of two possible design pathways for efficient conversion with higher power density - (i) high-frequency cascaded two-stage AC-DC conversion, and (ii) single-stage AC-DC conversion at a relatively lower frequency. The detailed system modeling and design methodology for the two converter topologies are laid out. As one of the key contributions of this work, a direct comparison of the two converter concepts based on achievable volumetric and gravimetric power densities and efficiencies has been presented. Using the developed model framework, an extensive analysis is done to gain greater insight into the effect of key design parameters on efficiency and power density. As a proof-of-concept, experimental results are presented to validate the theoretical predictions.