If you have 4 nights, you could do both and Samana. You would likely go through Samana to get to Las Galeras anyway. The whole peninsula is gorgeous and in my opinion, the most beautiful place on the entire island. I loved the drive along the coast to Las Terrenas from Cabarete, and somewhere on that north east shore, we stopped at this beautiful beach that had a huge parking lot, some development (bathrooms, change rooms), and colorful cottages just off the shore. Playa Bonita maybe, but that is near Las Terrenas. Don't quote me on that. I have pictures, and should upload to confirm the name. It was picturesque, seemed relatively new-ish, and very quiet. Worth stopping off there on your way through.
We took the trip you mention in 2015, and I remember the vibe in Las Terrenas was similar to Cabarete. Lots of expats (mostly French), restaurants lining the shore, kind of artsy even. Everyone drove ATVs on the road between the restaurants and the beach.
I was in Las Galeras in 2013. At that time, it was a sleepy little Dominican fishing town. The people were extremely kind and helpful, and it definitely didn't have an expat influence at all back then.
If you are a beach person, Playa Rincon. If you are the adventure type, Playa Fronton. I liked the El Limon waterfall, but had only been to Damajagua prior to that, so may have been a bit starry eyed.
If you are a budget traveler, I can recommend a great place to stay near El Limon. If you are not a budget traveler, but like adventure, I always though it would be cool to stay at the Dominican Tree House Village near Samana.
