Polymeric Insulator Ratings

Assuming power frequency voltage and either the same insulator or both having the same strike and leakage distances (with similar end caps), similar electrical field and ground plane clearance, the flashover voltage will be higher on the vertical insulator than on the horizontal. The vertical position is simply more efficient.

That's the primary reason horizontal deadend insulators are usually longer than the tangent insulators on the same transmission or distribution line. Typically, when the same porcelain cap and pin insulators are used for both tangents and deadends, the deadend string will usually contain one or more insulators than the tangent strings. The amount of additional insulation depends on the voltage and environment.