said by doechsli:
I think foam is the best insulator per inch but once insulated, you can't ever fish a wire again in case the need arises.
It doesn't have to be one or the other. Many people spray one inch of foam and then use fiberglass. That adds R-value as well as seals the house to air infiltration while still allowing the potential of pulling new wires. However, in my experience, you don't pull wires in exterior walls. You almost always have to cut the sheetrock as there is no attic access to drill through the top plates. Worst case with a foam filled wall is you have to do some foam carving to install the new wire. Not that big of a deal.