Use whatever fits the best. A HEI coil develops a minimum of 50KV open circuit voltage, so a difference in material on the end of a wire will never have any practical effect on the performance of the system. Clean spark plugs with the proper gap fire at <10KV so you've got at least 5 times the voltage needed for a good spark at your disposal.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!