Abstract: Lightning strikes on transmission lines are one of the main factors affecting the reliability of power grid operation. To balance lightning protection effectiveness and economic efficiency, metal-oxide surge arresters with external series gap (EGLA) have been gradually widely adopted in overhead lines. Commonly used EGLAs can be divided into two types: those with an insulator-supported gap and those with a pure air gap. Each type has its own characteristics in terms of electrical and mechanical performance, particularly their differential performance across various voltage levels, which has not yet been systematically evaluated. This paper conducted a comparative experimental study on these two types of EGLAs based on four voltage levels: 35kV, 66kV, 110kV, and 220kV, focusing on the analysis of lightning impulse insulation characteristics and mechanical performance differences. The results indicate that both structures meet the requirements of the standards at different voltage levels. In terms of electrical performance, the supported gap structure exhibits overall higher voltage-time characteristics due to electric field distortion caused by the insulating material, along with a stronger polarity effect. The negative polarity discharge voltage is significantly higher than the positive polarity, with a difference of up to 17.3% especially at the 220kV level, indicating certain protection blind zones. In contrast, the air gap structure has a more uniform electric field distribution, flatter voltage-time characteristics, a weaker polarity effect, and an average reduction in negative polarity discharge voltage of 14.7%, demonstrating superior protection capability against bipolar lightning strikes. Regarding mechanical performance, the requirements for the two gap structures differ significantly as specified in GB/T 32520-2024.