The main difference is the platinum plugs (factory standard) are suppose to last 100,000kms, whereas the copper ones are 10,000kms.
(The platinum electrodes dont get eaten away as quick as copper so last longer..)
I replaced my platinum plugs at 75,000kms with copper ones and gapped at 0.9mm due to running higher boost, and i havent had any problems at all. That reminds me its probably time to change them...
If you dont mind changing more often get the copper ones...
i like copper ones because changing more often you can see the state of each cylinder by the plugs condition (too lean, too rich etc)
Oh yeah, what gtst said the copper ones are suppose to have slightly stronger spark.