Mike's right, unless there is something wrong with your stock ignition...you wont gain anything from the nology. The stock system has plenty of spark to fire just about anything you can throw at it.
I have them both on my bike and the Iridium plugs and I have zero problems. I cannot tell you if there is a difference between stock and Nology because the dealer put them on as soon as my bike came out of the crate along with the British Customs Performance Package, tach and center stand. The tech guy at the dealership said it was the best Bonneville he had ever setup and pre-ridden the first 3 miles. So big deal . . . what did I waste on it? Maybe an extra $200 all in with the Nology and plugs. That was a long time ago and I was on the fence much longer with much larger choices later on!
Hedge's words exactly describe most of the Nology purchasers, "works great, no problems here" but not anything that really helps argue for them rather than being nice replacements if your stock coil dies.
-Ben