you will get a lot of difference and the same answers on this (confused yet LOL)
couple of weeks ago i installed some stainless shorties, installation went great, we heated them up and cooled down several times and retorqed several times. we later retorqed again and they were still tight. if they are designed for the vehicle they will be emission legal. ceramic or stainless is the only way to go for longevity. sound did not change much, power on the bottom end had definately increased, haven't refueled yet, but it seems my mileage will increase (will keep an eye on that benefit)
so for MY personal opinion, headers are worth it.