Our big green style, cannot be defeated!
-Sgt. A.J. Johnson - Halo 2 - Metropolis
To me, it seems that the necromorphs are more easy to contain than the flood. Also, you need a spine & brain for the flood to control you, so assuming that all the necromorphs still have those, a flood spore may still be able to take control of it. I'm giving it to the flood, only because they get smarter. They know how to repair ships, use weapons, & overall, solve problems. An infected flood host also gets stronger with the parasite. Both the necromorph & flood are rotting bodies though, so they can easily smash each other.
So, all types of necromorphs versus all types of flood (The ones from the books as well as pure forms)? The flood would probably win.
So think about a flood spore infecting a human or sangheli, it would aquire that organisms memories. Now, if they form into a giant mass, with all those aquired memories, they form a large mass called a gravemind, which is smart as -blam!-. They all coordinate by being controlled by that gravemind. There can be more than one gravemind.