I've got a bucket of old plugs (use them for fishing weights - another story) and most are AC some Autolites and a few NGK and Bosch. I've been pleased with AC in my Chevy's, but I always have always used NGK in the two strokes and race bikes. Ford uses platinum in the 5.4 Expedition and they performed well for 120K miles - looked like they could go a lot longer, but mostly highway driving.

Never had problems with the NGK and believe me, we've had our share of mud with the motocross bikes over the years!