Aesthetics. A vertical array will have better off-axis performance although you probably don't have seating far enough out.
A speaker with identical drivers, cross-overs, baffle width, and enclosure volume will provide the best match and not have anomalies where a sound track spreads music across the front 3 speakers.
A speaker with identical drivers, cross-overs, baffle width, and enclosure volume will provide the best match and not have anomalies where a sound track spreads music across the front 3 speakers.