As you say, both beaches are close, basically on either side of the town, both big and pretty. The semi circular Playa de Ris is a blue flag beack, and is popular with surfers. It's wide at low tide, and the right hand side is quite protected by the island and rocks there. Playa de Trengandín is a really long beach, 6km, is quite calm, but is quite rocky in places, as you'll see at low tide, it's very pretty too. This website might be useful as a guide, put it in google translate maybe: http://www.noja.com/s_playas.asp
Will you have a car? There are lots more lovely beaches easily accessible along this coast if you've got a vehicle.