My experience is you get diferent benefits from over under and side by side placement of dual centers. Over and under helps center the dialogue on the screen vertically which can be a problem for some people’s seating arrangements with a single center speaker.
Putting them slightly apart side by side can help spread out the center channel when using a very wide screen relative to seating distance.
As you already suspect which will work better is dependent on the situation and personal preferences. What size and aspect ratio is your screen? How far is your seating? How far off center are your farthest seats out to each side?
