Submarines and missiles reinforce the same sea-denial problem

Will submarines or missiles have the greater effect on future warfare at sea?

Both: submarines create persistent uncertainty and can launch missiles, while missiles can also attack from many other platforms.

Submarines and missiles contribute different parts of sea denial. A submarine changes behavior even when it never attacks because an opponent must avoid or search areas where it might be present. When the boat is actually there, that uncertainty becomes a direct threat to ships and logistics. The submarine can also carry missiles, combining concealment with long-range attack. Missiles remain independently important because aircraft, ships, and land batteries can launch them as well. One complicates where a fleet can move, while the other multiplies the directions from which it can be struck.