Hiriketiya and Mirissa come up together in almost every planning conversation about the south coast, and they are often presented as similar options. They are not. They have almost opposite atmospheres and attract very different travellers.
Mirissa is the more famous of the two. It is known for whale watching between November and April, Coconut Tree Hill, and the social beach scene along the main strip. The beach itself is beautiful. The restaurants on the beachfront are expensive and aimed at tourists. The evenings can be loud. Mirissa suits people who want whale watching as a priority and a lively beach atmosphere around it.
Hiriketiya is a small horseshoe bay about 30 minutes east of Mirissa toward Dickwella. It has a consistent left-hand surf break, a collection of genuinely good cafes, no nightlife to speak of, and an atmosphere that is deliberately slow. Most people who come to Hiriketiya spend the day surfing or swimming, eat lunch at one of the beachside spots, read or work in the afternoon, eat dinner early, and sleep. That is the rhythm of the place and it is one of the things that makes it distinctive on a coast where most towns are trying to be busier rather than quieter.
Who each one suits. Mirissa suits first-time visitors who want whale watching and a recognisable beach holiday. It also suits people who enjoy a social atmosphere and are happy to pay tourist prices for the convenience of everything being on one street.
Hiriketiya suits people who surf at an intermediate level or above, people who have already done the busier south coast towns and want something quieter, and people whose ideal beach day involves good coffee and no agenda rather than organised activities and beachfront bars.
The honest answer for most travellers is that Mirissa works best as a one or two night stop, particularly for whale watching, rather than a week-long base. Hiriketiya rewards longer stays. Three to five days in Hiriketiya is where most people find their rhythm with the place. If your trip is long enough, both are worth doing in sequence rather than choosing between them.