Aluthgama Hikkaduwa belt or deep south Mirissa Tangalle in late May

In late May both belts get the southwest monsoon, but the deep south (Mirissa, Tangalle, Hiriketiya) often gets slightly less rain and rougher sea than the Aluthgama-Bentota-Hikkaduwa belt which is more exposed to direct southwest swell. So if you must pick a south coast base in late May, the deep south is marginally better and far less crowded. Vibe difference matters more than rain: Aluthgama-Bentota-Hikkaduwa is the older "resort hotel + watersports" belt, more package-tour feel; the deep south is the younger surf-and-yoga scene with boutique villas, beach bars and the Hiriketiya horseshoe bay culture. Honestly though, if you can shift to the east coast (Arugam Bay) for late May you will get sunshine while both south belts are getting monsoon rain.