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The SAE has a book published that is all about spark plugs and the tests that they have done to test every kind of spark plug that they could find, it boils down to the fact that a spark jumps between sharp points or edges best and has a harder time jumping to a rounded surface, that is why it's not a good idea just to regap a worn spark plug.
The larger the gap the more fuel the spark is exposed to and the faster the flame spread will be but to large and the spark gets weak and inconsistant, the smaller the gap the stronger and hotter the spark will be, so you want a gap in the middle, large enough that the spark is exposed to fuel but small enough that it's a nice strong hot spark.
Platinum is hard and resists electrical wear, it is also a poor electrical conductor and expensive so they only put a thin layer of it on, I've also heard that there is only a few cubic yards of platinum on earth, so because of platinum being a poor conductor you get a slightly weaker spark, but because platinum doesn't wear very fast a spark plug can last many times longer, so if a copper spark plug needs to be changed every 15,000 miles, it might be safe to say that a platinum plug will perform about the same as a copper plug that has 10,000 miles on it but the platinum plug will last 100,000 miles and the platinum plug will perform the same over most of it's life.
There are also silver core spark plugs, silver being a better conductor then copper and from what I've read they really are better, but they also cost a lot more so you might never see the return in your fuel savings but you might see a slight boost in Horse Power if that slightly hotter spark is what you need.
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