The beach in Las Terrenas is much longer, about 30 km, so there are fewer people in any particular part of the beach. But I wouldn't say it is nicer.
If you are looking for a school were you can learn Spanish, or build on the Spanish base you already have, I think Sosua is a much better choice that Las Terrenas.
An option to living in Sosua is to live in nearby Cabarete. Great beach there, and a nice laid-back atmosphere. Easy to get back and forth between Sosua and Cabarete by public transportation. Only 7 pesos each way, which at present rate of exchange is about 35 cents US.