Headers vs 360 manifolds for 318

I've run both, many times. Without doubt, the manifolds are better fitting, and less trouble all the way around. No, they don't perform as well, even on a mild 318. Let me offer this: IF you are going manifolds, get 360/340 B-body manifolds from around '72-73. Very cheap, and do not raise up over the steering box. Much easier to change plugs or anything else on the driver's side with these manifolds. Lately, I've been just ordering the TTI head pipes for them. A nice combo for a mild motor daily driven.
You can go to the "2018 small block race" in the race forum and see the manifolds used on both my Dusters.