We present a detailed comparison of the optical properties of a quantum well (QW) and a two-dimensional (2D) quantum dot superlattice (QDSL) made of In0:4Ga0:6As on GaAs(311)B substrates under the same growth conditions. The lowest electron to hole transition energy in strained In0:4Ga0:6As/GaAs(311)B QWs is calculated. While good agreement between the calculated transition energy and the measured photoluminescence (PL) peak energy is found in the QW, the PL peak energy of the 2D QDSL significantly shifts to a lower energy with respect to the calculated transition energy for a QW with the same nominal thickness. Based on the linewidth broadening induced by increasing excitation density and temperature, the exciton-exciton and exciton-phonon interaction strengths in the 2D QDSL are found to be much larger than those in the QW. As the exciton-phonon scattering is sufficiently strong, a transition of the exciton motion from coherent to incoherent occurs.