Understanding detailed avalanche mechanisms is critical for design optimization of avalanche photodiodes (APDs). In this work, avalanche characteristics and single photon counting performance of 4H-SiC n-i-p and p-i-n APDs are compared. By studying the evolution of breakdown voltage as a function of incident light wavelength, it is confirmed that at the deep ultraviolet (UV) wavelength region the avalanche events in 4H-SiC n-i-p APDs are mainly induced by hole-initiated ionization, while electron-initiated ionization is the main cause of avalanche breakdown in 4H-SiC p-i-n APDs. Meanwhile, at the same dark count rate, the single photon counting efficiency of n-i-p APDs is considerably higher than that of p-i-n APDs. The higher performance of n-i-p APDs can be explained by the larger impact ionization coefficient of holes in 4H-SiC. In addition, this is the first time, to the best of our knowledge, to report single photon detection performance of vertical 4H-SiC n-i-p-n APDs.