Surely the flattened seventh came first, because that is the normal note in the myxolydian mode. However, some diatonic instruments can’t play a flat seventh note .or chord easily, so the tune is often adulterated in deference to squeeze box players, etc. In fact, many play it in the key of D, so they can get the flat seventh by playing a C natural. The same goes for contemporary versions of Horses Bransle and the like, which in old notation has a flat third note, but is seldom now heard.