Headers are a pain in the butt. First time you catch one on a railroad track, you really wonder if they are worth the effort.
I watched a video from Nick's Garage once and they did a worked over, stroked 383 with 440 source heads and they came up with ~20 HP and ~20 ft/lb torque gain. Decent gain, but they didn't show what the lower RPM (street use) gains/losses would be. It was also run with no mufflers, so (IMHO) that might make the difference less.
So.. Is it worth the HP gain at high RPM? Street car? I don't think so. Race car? Yes.
I've run headers and then switched to manifolds on my A12 Roadrunner. The headers drove me nuts and after changing a starter I decided to take them off. The car ran just as fast (seat of the pants dyno) with the manifolds as it did with the headers and quite frankly, I never had to worry about hard to change spark plugs, burnt wires and leaks.
IMHO, I was always happier with cars I didn't have headers on. As I said, catching a railroad track a time or two (seen a lot of other guys do it too) really takes the fun out. I really think that "on the street" the manifolds don't give up much to the headers. I also think that on the low end, the manifolds perform as well or even better in the low RPM range where a street car is used.
Your choice though... That's my opinion and when I've brought up these facts as I see it, I've been called a "purist". You have to weigh the gains versus the effort and $$ involved.