I've got some Philips X-Treme Power lamps in the car, they replaced some Osram Silverstars.
Silverstars were a good improvement on the originals when they first came out, and the X-Treme powers were a similar step up again. From what I've read, there's little to choose between Nightbreakers and X-Treme Powers, I'd go for whatever is cheaper (
Powerbulbs.co.uk are a good reliable supplier, but they also have an ebay shop where prices are usually better). Usual warning, there are some fakes on ebay.