An experimental study was conducted to compare laser ignition (LI) and spark discharge ignition (SDI) in an ethylene fueled supersonic combustor operating at a global equivalence ratio of 0.23. The Mach number, total pressure, and stagnation temperature of the inflow were 2.92, 2.6 MPa, and 1650 K, respectively. The flame kernel generated by the laser pulses experiences a rapid growth in size when it propagates towards the cavity leading edge and resides there. The cavity recirculation flow plays an important role at this stage because the velocity of it far outstrips the turbulent flame speed. In comparison, the ignition process of SDI is significantly slower because the long pulse duration of the spark discharge decreases the energy density of the plasma and makes the flame kernel more vulnerable to flame quenching. The extra heat loss resulting from the electrodes also delay the ignition process. After reaching the cavity leading edge, the flame kernel forms a small self-sustained flame there. Since the plasmas have transformed into flames, LI and SDI are similar in the aspect of flame propagation. The competition between the chemical reaction and the losses of heat and radicals determines the evolution of the flame. Prior to the reaction zone, the hot products generated by the flame are transported downstream via the cavity shear layer, which reduces the ignition delay time of the fuel/air mixture and promotes the accumulation of radicals in the flame base. The enhanced flame base further accelerates the propagation of the cavity shear layer flame. At the rapid propagation stage, the cavity shear layer flame spreads downstream prior to the flame base because the cavity shear layer dominates the evolution of the flame.