Both bus trips are a good 3 hours and 30 or 40 minutes. They are virtually the same cost and time. Both buses are very very cold - air conditioners blast. When I choose one over the other it is almost always about where I want to either get the bus in SD or get off the bus in SD. Metro terminal is in Piantini area and Caribe is more in the core area - don't know what it is called.
4,500 RD seems a bit high for this trip. We just had a taxi go from Puerto Plata to Juan Dolio (45 minutes the other side of SD) and return for 6,000 RD. However a night time trip will cost more.
Make sure to negotiate as Canada Bob says. It is a beautiful trip and should take about 3 to 3 1/2 hours depending on time of day and traffic conditions.