[darkpuppet]

I would say stay away from the copper plugs unless you're running boost.

a ) they're made out of copper, and should be replaced every 30,000km/mi
B ) they're meant to run cooler, which may hurt performance and mileage if you're not boosted. By running cooler, they avoid pre-ignition/detonation that would otherwise be caused under the increased strains of boost. However, under stock conditions would result in a possibly slower ignition of the mixture/delay the flame front.

best to stick with the stock part when changing your plugs. They do the job when used as designed by the ford engineers.