Three things: light direction, season and pace. Koh Samui faces east; its finest light is in the morning, not at sunset. Its dry season (January to September) is the opposite of Phuket’s, which means they are genuinely complementary choices. And Koh Samui is quieter, more intimate and less internationally busy which produces a different atmosphere in the photography. Neither island is better; they suit different couples and different aesthetics.