Exhaust Manifolds

I guess I will carry the pro manifold torch here. There are two things manifolds have over headers. Simplicity of installation/maintenance and ground clearance.
The installation/maintenance is pretty clear. There is a lot more room into the engine compartment for one large pipe on each side of the engine than four smaller pipes. That makes access to your starter, oil filter, clutch linkage, steering, brakes, and suspension so much easier.
Ground clearance may or may not be an issue with modern headers. Be careful about what you are shopping for. All I know is that I bought the cheapest set of headers for my Duster back in the late 90s and they lasted for less than a year. They caught on speed bumps, trailer transitions, and even regular bumps. Hard to brag about performance when your bottom two tubes on each header are smashed flat. I'm told the expensive headers like TTI and Doug's have solved that issue. All I can say is that if your headers go under the steering linkage, be prepared for low clearance issues.
In short, my opinion is this. If you want your car to be a no fuss, go anywhere type of cruiser/ daily driver, I'd consider manifolds. Not necessarily 340 manifolds. They are not worth the money IMO. If you're looking for performance, go expensive headers. You'll probably spend as much money and get more performance. Of course, if you want to sacrifice a bit of convenience for performance, headers are a great way to go. Just be aware what you are sacrificing no matter what the choice you make.