Headers will give you better performance because of the design. The sweeping curves allows the gas and air to enter the firing chamber because any sharp bend in exhaust manifold will cause the heaver gas particles to sling out of the air stream. The better vapor condition the gas retains, the more power and better performance you will get. The problem with headers seems to be keeping them tight. And, they have a tiney sound that some people like, but I don't care for. The split manifold would have a better sound. I have always wondered if headers will transfer heat away from the head as well as a cast iron manifold and maybe have pre-mature valve problems. This is my opinion and concerns, maybe someone can help you further.
Truck Tinker
