We did this trip (Sosua to Sanchez and then taxi to Las Terrenas) in December. We found it very comfortable and convenient (and CHEAP). The buses are similar to a greyhound or voyageur, but with a little more wear and tear.
I know I have mentioned this before, but it bears repeating. If you get up to use the washroom, hangon tight to anything that you can get a hold of at ALL TIMES. I failled to do this, and if my husband hadn't been standing up to let me into the window seat and grabbed me, I really could have gone flying. As it was, I stubbed my toe and cracked the nail about half way down. (try walking around in sandals or flip flops without flexing your big toe LOL).
I really enjoy the ride to Samana. I spend the whole time with my nose pressed against the window.
The ride from Sanchez to Las Terrenas is BREATHTAKING (in so many senses of the word). My personal favorite is the German castle that is being built on top of a big hill.
Depending on where you are staying, I would suggest that you make arrangements to have them have a driver meet you to take you to Las Terrenas or Las Galeras. Less confusion and you are more likely to get a good price and a reliable and safe driver.
Rose