Las Galeras and Las Terrenas are two VERY different places and I enjoy both. I have lived outside of Las Terrenas (3 miles) for almost 7 years and in all honesty, don't think I could live 'in' the main town.
Las Galeras is very quiet with very limited resources. No large supermarket, one gas station, small ferreterias which all mean you need to travel to at least Samana for many things. Many people do a monthly shop in the supermarkets of LT!
But it has some gorgeous beaches, is quiet, some nice bars but can be very, very quiet in the off season.
For me a place to day trip, maybe overnight in.
Las Terrenas is much busier. Now has three large supermarkets, lots of ferreterias, lots of resources and unfortunately, a shite load of building going on so lots of dust and big MAC trucks which are a right royal PITA.
I go into town when I need to, but generally stay on the outskirts.
There are lots of places to eat in and around LT and lots of nice beaches and hidden spots and lots of night life.
That said, its also busy as hell with internal tourism on big holidays (lots of expats leave LT for Semana Santa!!!).
From LT, 30 mins will get you to some varied beaches on both side of the Samana Penisular (Los Robalos is gorgeous).
So it's down to personal choice and what you really want.
IMHO, if you want to be surrounded by a large Expat community, then LT is a better choice. If you like peace and solitude, Las Galeras.
Oh and a last thought, if you have kids, LT has the better schools.