HD 10R12As is what you should be using.
Recall notice here -
http://www.ukbeg.com/pash/B029-263576.pdfQuote:
Use only 10R12 spark plugs as replacement spark plugs for all model year Buell motorcycles. The 6R12 spark plugs could not be superceded because they are still used on Harley-Davidson Sportster models.
Plenty of discussion on NGK Iridium plugs, some say they make their bike run better (wouldn't you if you had spent lots of money on them), however others say they make no difference and others say they are more prone to fouling....