[ygra]

At least from Rostock to somewhat closer destinations you have both options. There's a bi-hourly IC to Hamburg or Berlin and another bi-hourly RE towards the same destinations. They're not terribly different in terms of travel time, but one is a regional train and one is an inter-city train.

Sure, long distances (I had to travel from Rostock to Tübingen last weekend) are typically not taken with regional trains (although you technically can; I did that as a poor student a few times, it just takes 16 hours instead of 10), but over medium distances (around 2–3 hours) you often have both options.