Unawatuna or Mirissa — which should I visit?
Unawatuna suits calmer swimming and a family-friendly beach close to Galle Fort. Mirissa suits whale watching and Coconut Tree Hill. Both are close enough that many travelers visit both.

Two real south coast beach towns, compared for swimming, whale watching, and pace — both within a short drive of each other.

South coast beach base near Galle Fort, Dalawella, and Jungle Beach.
South coast beach town known for whale watching, Coconut Tree Hill, and Weligama coast access.

Unawatuna — generally the calmer of the two, and tagged as family-friendly.
Mirissa — the south coast's main whale-watching base.
Unawatuna is the closer of the two to Galle Fort.
Close enough that a private driver can combine both in a single day trip along the coast.
Unawatuna suits calmer swimming and a family-friendly beach close to Galle Fort. Mirissa suits whale watching and Coconut Tree Hill. Both are close enough that many travelers visit both.
Yes — a private driver can combine Unawatuna and Mirissa in one day, since they're both on the same stretch of south coast.