Re: exhausted...what\'s the diff?
Headers: Reduces pressure of exhaust coming out of the engine. Allows the engine to "breathe more free". Gives more power at higher RPM's. Welded tube construction.
Exhaust manifold: Stock setup usually, keep a little more backpressure, better for low RPM power. Cast iron construction.
Personally on a 4.2L engine, I like to run a stock exhaust manifold because the 4.2 has most power at low RPM's, where the stock manifold is strongest. Headers are for high RPM breathing, this is where my 4.2L does not often rev.
Also, the Header causes higher underhood temperatures (fact). Stock manifold is cheaper. Headers often cause clearance problems to steering shafts, auto transmissions, linkages, etc. This was also a problem for me. Finally, it is often harder to get a good seal with aftermarket header flanges in my experience.
I bought my used exhaust manifold for $15, had it resurfaced for $20 (required) at a machine shop, and then used a high temp bake on coating which looks like raw cast iron.