Just a follow-up for those who might be interested...

I work at DaimlerChrysler and talked with the engine lab tech guys on spark plugs.

Platinum and Iridum:
What I learned was that irregardless of the manufacturer, Platinum and Iridium plugs are designed to meet modern OEM 100,000 Mile Service Requirements. The primary reason is due to their excellent anti-corrosion properties.

These types of spark plugs were designed with extremely fine tips to maximize emissions/fuel economy in modern injected engines where the gas/fuel mixture is optimized.

In general, these type of spark plugs perform poorly in carbureted cars... especially those that are running rich.

Silver:
Irregardless of the spark plug manufaturer, Silver plugs have much better a significantly larger tip and better conductivity characteristics than Iridium and Platinum. Consequently Silver Plugs will always out perform Platinum or Iridium in carbureted cars... again especially as these cars tend to perform better when running slightly rich. The disadvantage to Silver is Life Cycle... they do not have as good corrosion properties and will need to be replaced more often... also tend to cost a bit more.

At any rate, as an engineer I thought this was fairly interesting.
Hope others find this helpful, John