I'd recommend a center channel over the subwoofer if you're going to have more than one person watching movies/TV and if your main left and right speakers are far enough apart (i.e. your display is large, or you are sitting close to it) that the people watching will hear things coming from the left or right instead of center of the screen. This is usually more distracting than a lack of bass output.
A center channel won't necessarily improve the clarity of dialogue or other sounds, especially if the room is very echoey or if the center channel ends up needing to be placed inside of a cabinet or in some other way that creates additional reflections or increases distortion. Room treatments, or at least filling up the room with stuff, is the best way to improve clarity.
A center channel won't necessarily improve the clarity of dialogue or other sounds, especially if the room is very echoey or if the center channel ends up needing to be placed inside of a cabinet or in some other way that creates additional reflections or increases distortion. Room treatments, or at least filling up the room with stuff, is the best way to improve clarity.