Power transmission networks play a critical role in linking generation and distribution systems. One key aspect of the network's performance is voltage optimization. This study focuses on comparing the impacts of High-Voltage Direct Current (HVDC) transmission and Flexible Alternating Current Transmission Systems (FACTS), specifically the Unified Power Flow Controller (UPFC), on system voltage stability, grid power losses, and transmission capacity under load fault conditions. This present study develops the IEEE 30-bus and IEEE 57-bus systems as test cases, incorporating Genetic Algorithms (GA) to analyze the effects of HVDC and UPFC integration. The Power System Simulator for Engineering (PSS/E) version 33 software program is used to model multi-terminal UPFC and HVDC. A comparative study is performed between the system's performance with and without HVDC and UPFC under various load conditions in the transmission network. Three load conditions were analyzed. The results demonstrate that for the IEEE 30-bus system, the total active power loss under normal load conditions is reduced by 69.594% after adding UPFC between buses (3-4) and by 75% after introducing multi-terminal VSC-HVDC between buses (2-6) and (2-4). Similarly, reactive power losses are reduced by 74% with UPFC and 73% with multi-terminal VSC-HVDC under the same conditions. For the IEEE 57-bus system, the addition of UPFC and VSC-HVDC improves active and reactive power losses by 49% and 55%, respectively, under normal load conditions. The studied results confirm that connecting HVDC to the system achieves better results in terms of bus voltage profile, a significant reduction in total network power losses, and a higher effective power transfer rate compared to UPFC. Moreover, multi-terminal HVDC transmission delivers greater voltage improvements and larger reductions in total power losses compared to adding UPFC to the same system.