So here's my experience. Jet Hot is down the street from me, and I've used them every time I've needed to have headers coated. They happen to have my Red Iroc with ducati bronze headers in their gallery btw, ahem (shameless plug).
...but seriously, they coat the inside, and I've never had problems with coating or headers deteriorating. I've had others that came coated, and can't think of an instance when I have NOT had problems. Some worse than others. Now, do I think Jet Hot is the only game in town? Of course not. Are they arguably pricier based on their name? Haven't shopped, but I wouldn't be surprised. All I know is that in my experience, a reputable coating, inside and out, seems to be not a bad way to go. Not expecting everyone to run out the Jet Hot, but I'm building a new mill for my 86 Z, and the headers are coming coated. I'll actually pay Jet Hot to strip and recoat. Just not worth the hassle down the road. jm2c.