In Tangalle: Ganesh Garden Cabanas is THE place to stay and never leave.
It's the only place for miles around on an enormous beautiful deserted sandy beach. The 10 or so bungalows there are brand new and clean and cost around 10 euros per night. The food is excellent. The management is very friendly. A tuk-tuk from downtown Tangalle will cost around 1 euro.
Unawatuna?: Better deals for guesthouses can be found just a block or so set back off the beach. It is also much quieter, so on nights when they crank up the rave music on the beach until 4:00 AM, you will still be able to sleep like a baby, unlike those who paid more to look out the window at the sea. Try "Rock House" for big cheap rooms with private terraces. We enjoyed the monkeys hanging around there which was cool.
Food In Unawatuna?: Definitely eat at "Hot Rock" breakfast, lunch, and dinner.