They are both very good plugs, the agents sell both as well.
I'm slightly biased toward NGK for outright quality, but Bosch have come up with some clever designs as well recently.
I would suggest buy a plug based on your driving style, some perform better flat out or highway driving whilst others resist carbon fouling and provide good start and cold running characteristics. The overall performance difference is minimal between different types anyway.
The new technology in ignition systems these days seems to be four sequential sparks in one millisecond, aparantly this will ignite just about any mixture.
