I just pulled my motorcraft platinum plugs yesterday (original after 63K miles) and the electrode was very worn and melted. I sent Alberto a picture and he said that i would greatly benefit from replacing them with the cooper plugs that are one heat range colder. Hope that helps some with your decision.I had purchased Autolite DP's to install (to replace the stock Motorcraft platinum plugs) but Alberto (americanmotorsport.com) advised me not to install them or any other platinum plugs as I am getting ready to purchase a Mircotuner/chip from him.
He highly suggested Motorcraft Copper Core plugs one heat range colder then stock for my 98 Duratec. His reasoning follows.
Copper is a much better conductor of electricity, so it will in turn provide for a better spark. The reason the factory uses platinum, is simply for durability, as you have to change copper plugs much more often (30k vs 100k). People have, over time, begun to THINK platinum's are better...they are not.
One heat range colder than stock is better. The OEM tries to get the best emissions possible, so they try to run the cylinders hotter. If you run a heat range colder, you are in turn extracting more heat from the combustion chamber, and you can run a bit more aggressive of a tune, and guard against detonation.
They recommend Motorcraft, because they have seen the best results from them out of them all on the dyno and on durability testing.
I've had platinum plugs before for other vehicles but never noticed any noticable performance increase or decrease. However I'm going with Alberto's recommendation on this one as I trust his experience and all the recommendations he has received on this site.