I think it boils down to goals & priorities you choose when building your car. Generally speaking, when it comes to getting the most horse power from your engine, headers are the way to go. However, any HP gains from adding headers is going to be offset with other possible factors as well. Headers are louder (which is why I think most people want them, the intimidation factor) generally produce more under hood and floor pan heat, (even coated or wrapped examples) often reduce ground clearances, can interfere with other under hood or chassis componants causing rattles, rubbing, etc. and sometimes can be leak prone especially at the head surfaces. Not to mention, if you have a newer OEM style exhaust system it is often required that you modify it to mate with the headers, not a big deal but your OEM exhaust from the mufflers forward is now ruined.
Most Mopar HP exhaust manifolds are quiet, less likely to leak, make excellent horse power, run cooler under the hood, and are nearly as valuable used as when new. Also, keep in mind that you can only use a certain amount of HP on the street, traction being the limiting factor, you may gain a little HP but loose it in tire smoke just as easily. At the track with slicks you may gain a tenth or two, but on the street you probably won't notice any real difference other than the added rumble from the headers. After many years of driving cars with both headers and stock manifolds I like the manifolds.
