Nick, do you have anything to add to my post on the plugs? I'm curious and want to get back on topic.
For the sake of headers, the differing expansion rates of different metals and actual engineering of a header will increase the "scavenging" effect and make the engine more efficient as an air pump.
For the sake of daily driven Accords, I don't think it makes a difference. Yes, some cheaper headers hold their own for a LONG time. At the same time, we've all heard the horror stories of cheap headers breaking welds. Across the entire automotive industry and aftermarket, you get to a certain point where you're only paying for materials, labor, and packaging. This is why those cheap headers suck. You aren't paying for an experienced journeyman TIG welder to weld this stuff, you aren't paying for research and development, etc. Quality stainless steel is expensive, fully penetrated welds aren't easy and are very time consuming. How many times do we have to say it on here. YOU GET WHAT YOU PAY FOR.