There is actually express bus to Higuey. Does not cross San Pedro nor La Romana.
Leaves every 30 minutes.
The non-express buses cross San Pedro and La Romana and leave every 15 and 45 minutes of the hour.
Service is basically the same, TV/aircon.
Express bus ticket you buy before boarding a bus, non-express you pay on board.
The time difference in travel may be 10, 15 minutes max.