Either Metro or Caribe Tours
Both run good service to Santiago and Puerto Plata.
Trip was exactly 2 hours last Monday (SDQ-STI) But it took 40 minutes to get to the toll plaza! (5 p.m. traffic.)
If you are going to spend a night or afternoon evening in Santiago you must get in touch with AZB, our local pim....errrr guide to the night life....Anyway he knows his way around town.
I can show you the main parts of the town and share a beer with you, but I am obliged to be at home shortly after sundown...
HB
Edited to add: If you decide to stop over in Santiago, and take Caribe Tours, rather than Metro (I prefer Metro), be sure to get off at the first stop in Santiago, since the main terminal is way the heck on the other side of town....