SS is where the ferry comes in and is the island's capital. It is a lively place with a good range of shops, restaurants, and all facilities. There are two good beaches. The main bus routes all start and finish here so travel by public transport is easy. The parador is on the cliff top overlooking the town.
Playa Santiago is a smaller affair. Used to be mainly a fishing village by tourism is developing there now mainly fed by the HPB complex and UK visitors. The Tecina hotel is on the opposite clifftop and is delightful. It is about 40 mins by bus to SS and the ferry. The beach at PS is mainly rocky although a small sandy area is being developed now.
Id suggest SS for a first visit to Gomera.