Sorry it's taken me so long to respond. I just got home from a long trip. Bowed dulcimers have three or four playing strings of modern manufacture and easy availablility. They have metal frets and are relatively inexpensive when compared to tenor vios. Tenor viols have six strings on a fairly flat bridge. The strings are gut (costly and not so easily available). The frets are also gut and need to be replaced more frequently than the BD. Bowing the bowed dulcimer is easier because of the greater arch of the bridge. Each individual string is easier to get to and your bowing doesn't have to be so precise. Bowed dulicmers, being the shape they are, are easier to carry around. They are also generaly cheaper than tenor viols and tend to be more robust.
I patterned my design of the bowed dulcimer on the gamba and internally they are both very similar. I think of my bowed dulcimers as a modern version of the viol and I often play Early Music material on mine. Bowed dulcimers ar e also cheaper than viols and you have a greater selection of bows to choose from since most versions of the bowed dulcimer use a cello bow. If you have further questions, just ask. I'll be happy to throw in what I know.