7V or 12V Power Supplies for Arduino
The Arduino uses a linear regulator to convert the input voltage to 5V to run the logic. Any excess voltage gets converted to heat in proportion to the current being drawn (P = &Delta;V &times; I), therefore, you want to use the lowest acceptable voltage.

The regulator is physically very tiny, and has little if any effective heat-sinking to help it manage power dissipation, so the current that it can handle is very limited &mdash; basically the needs of the Arduino itself plus a few 10s of mA to operate external circuitry. Reducing the input voltage as much as possible allows you to get a few more mA out of it before it shuts down.