Fira is the capital and lively. It has great sunset views but not as iconic as what you'll see in Oia. Between the two, Fira is cheaper and from a crowds perspective, Fira is busier throughout the day but Oia gets insanely packed at sunset. However, what makes Fira ultimately the better area to stay in Santorini is that it's the central hub of transportation, shops, and other businesses which make it the most convenient.