Granted iridium plugs will last longer..and are more expensive..but which will run better ?

 

No significant difference at all, assuming you use a conventional design, single electrode plug. At least no difference until the gap starts to open up as the copper electrode (and its nickel/iron tip) erodes. Is your engine stock or modified?

My thinking is the extended life of the iridium, and it's ability to maintain the relatively large factory gap for many miles, is worth the extra cost. The NGK TR55-IX seems to be more popular than the AC Delco.

 

The only concern I would have is if you use or plan to use nitrous on your car. You have to pay close attention to the heat range when using these plugs to avoid melted electrodes. It probably has more to do with the relatively thin ground strap than matrial type, but I am not certain.

Otherwise they are a great plug and will,last 100,000 miles...

 

Go with the copper ones if you really enjoy climbing into the engine compartment and struggling to replace those hard to reach ones every 25k miles(?). Otherwise just do it once for 100k miles with the iridium ones.