sbeveraggi,
I would say SunByrne has summed up my personal opinion between the two quite well. I will add that the Sierra-1 NrT has a cleaner, more open and transparent midrange than the stock Sierra-1 and the new tweeter provides about as much sizzle and crispness that is possible without sound harsh. I would place it somewhere in between the stock Sierra-1 tweeter and the Studio 20 tweeter. It is not metal so it won't have the same amount of sizzle, but it also will not suffer from ringing, which is why metal domes -- even the best of them -- can sound fatiguing over time, especially at loud volume levels.