Yeah I mean doesn't really matter HOW you use both I just think unless you really want to use only one then why limit the options. I think having more than one style would probably add that much more depth to the experience. If you really did want to stay to just one or the other though the metal/orch one is a lot more lively than the metal only one.
Maybe you should go through parts of the game and get an idea how each one would fit and see if one works better than the other for each situation then decide how you want to proceed with it.