Hikkaduwa vs Mirissa for a first-timer with one week on the south coast - which should I choose?

I am based in Galle between both locations so I know them well. Here is an honest comparison for someone with one week and your specific interests.

Hikkaduwa:
- Strong point: the coral reef snorkelling directly off the main beach is genuinely excellent — you can wade in from the shore and see turtles, reef fish, and coral in waist-to-chest deep water. No boat required.
- Vibe: busier, more backpacker-oriented, has a strip of bars and restaurants along the main road. More affordable accommodation. More nightlife than Mirissa.
- Beach quality: the main beach is pleasant but gets crowded. Narigama beach (southern end) is quieter.
- Downside: busier roads, more commercial feeling, slightly less beautiful beach than Mirissa.

Mirissa:
- Strong point: whale watching from November to April. Blue whales and sperm whales are seen regularly from boats — it is one of the best whale watching spots in Asia. Also: the main Mirissa beach itself is more beautiful than Hikkaduwa — a curved bay with a distinctive palm-covered rock island (Parrot Rock) at one end.
- Vibe: slightly more relaxed and upmarket than Hikkaduwa. Good restaurants (Secret Beach, The Shady Lady, etc.). More couples and mid-range travellers.
- Snorkelling: you can snorkel at Mirissa but it is less interesting than Hikkaduwa. The reef is further out and less accessible from shore.
- Downside: no real nightlife compared to Hikkaduwa.

My recommendation for you: if snorkelling is your priority → Hikkaduwa. If you want the most beautiful beach and possible whale watching → Mirissa. For a balanced week with good food, some nightlife, and snorkelling → Hikkaduwa edges it. The two are only 30km apart and easily visited from each other as a day trip.