I agree with the other posters in that the part of Sosua you will likely be in has a strip called Pedro Clisante which turns into a "party town," complete with tourists chasing Dominican and Haitian hookers (or maybe it's the other way around.) That's the norm in that area, but don't think Santo Domingo - or any part of latin america, for that matter - is immune to it.
Santo Domingo is HUGE, noisy, and has crazy traffic, but is tops in terms of cultural offerings. It's hot during the summer, though, and if traffic is bad takes a while to get anywhere. Tons of nightlife (both kinds), and more than a few good restaurants.
Sosua is actually a pretty decent small town during the day, despite the squirrelly reputation. The local authorities are doing what they can to control the prostitution, and have largely succeeded in limiting it to the the above-mentioned Pedro Clisante area, and a few girls around the beach who are a lot more discreet than they used to be.
Sosua is part of what is sometimes referred to as the North Coast Corridor or Complex. It runs from Puerto Plata in the west, through Sosua, then on to Cabarete, Rio San Juan, and up through Cabrera. It's a bunch of towns of varying sizes that run across the coastline. There's lots of variety and interesting things to do there.
I guess my point is you'll likely find whatever experience you're looking for. Some want to work at a mission 26 hours a day, others want to chase hookers nonstop, and most want to do something in between.
That said, if you want to learn halfway decent Spanish, and enjoy typical NA or EU nightlife, consider Santiago. The weather's better, the Spanish is infinitely superior, real schools are abundant, and it's way cheaper to live there. Plus, the beaches of the north coast and the city lights of Santo Domingo are each only about 90 minutes away, so you get to sample both.