Copper is better than aluminum for electrical wiring because the electrical conductivity of aluminum is higher than that of copper. In the International System of Units, the electrical conductivity for aluminum is 3.5 x 10^7, whereas for copper, it is 5.96 x 10^7.
Electrical conductivity refers to the ability of a metal to move electrons along its surface with minimal resistance. The numerous tiny collisions these free electrons undergo impart an electrical current to the metal, which is what makes metals good for wiring. The best conductors possess a single electron in the valence shell. Copper has one, whereas aluminum has three.