Metro Tours is 30 minutes faster over Santo Domingo to Puerto Plata because they stop only once in Santiago and don't stop off the Hwy close to La Vega. They are considered by some to be cleaner too.
But they don't travel as often as Caribe Tours:
Metro Servicios Turisticos
If you want to Google Earth the locations of Caribe Tours and Metro, copy these coordinates into the "Fly to" window of your GoogleEarth program and hit "Enter".
Caribe Tours Santo Domingo: 18?28'33.93"N, 69?54'26.19"W
Caribe Tours Puerto Plata: 19?47'30.05"N, 70?41'52.74"W
Metro Santo Domingo: 18?27'52.96"N, 69?56'11.71"W
Metro Puerto Plata: 19?47'41.07"N, 70?41'12.83"W
Santo Domingo Las Americas SDQ Airport is: 18?25'43.78"N, 69?40'11.40"W
Puerto Plata's "Playa Dorada": 19?46'7.04"N, 70?38'36.83"W
... J-D.