What is more important for amperage rating wire size or insulation temperature rating?

Wire size determines how much current can be safely conducted. Insulation type and rating determines other factors, such as where the wire can be used and maximum voltage.

It should be noted that the high voltage lines that are hanging from the really large electrical towers are among a class of wires that have NO insulation around the outside of the wire. They are instead, connected to the towers with large insulators that prevent electrical energy from being lost through the towers.