Go with whichever one you feel most comfortable on. Although if the streets on your route to work are rough, the MTB would have a leg up. Many roads present tougher riding conditions than a lot of off-road trails. A mountain bike with thick tires was extremely useful when I lived in a neighborhood with brick streets. The streets between my home and my office are littered with potholes and rough pothole repairs (fortunately, I take a path most days).