Time has come to do plugs again, and I am torn between NGK and AC Delco. In the past I have quite often just gone the TR6 and replaced more often, but with the Iridium and Platinum plugs starting to become available for more competitive prices am not sure. So are the AC Delco for $60 a set as good as the NGK Platinum for $120 a set? Or should I just stick with TR6 and change more often. Car is VE Calais 6 litre with cam and decked heads.