Form-for-form, one necromorph of nearly any type is superior to a flood combat form. They can also bring out their bigger forms more quickly. The necromorphs can not only utilize weaponry (in a primitive manner), but they can physically fuse themselves to technology and use it in unorthodox ways, hence the twitchers. That said, flood are a much more [i]intelligent[/i] enemy, but the Marker's mind--blam!- powers would hamper that a good deal. That's actually extremely convenient for the necromorphs.