The right height of the end cap depends on the type of plant you intend to exhibit inside the end cap itself.
Tall Plants: to expose tall plants such as Kentia, Dracaena Lemon Lime or Strelitzia Nicolai, the 350 mm high end cap is ideal.
Low Plants: to expose low plants such as Poinsettias or African Violets, the 550 mm high end cap is ideal.