I have done both at times. Anecdotally, I have found both to be true. I don't have extensive experience working with women.
Women do tend to hold back a bit more when men are present. This seems to diminish over time. When I facilitate coed groups I tend to stay with behaviors/topics that are not gender specific or have polarizing effects. I would not recommend exploring behaviors related to sexual subjects or domestic violence. I can imagine these things are more effectively approached in a gender specific group setting. Although, I would not argue against it if the group was facilitated by an experienced therapist.
One significant advantage, I have experienced in coed groups, is that women tend to be more emotionally intelligent than many of the male clients. Men seem to pick up on this aspect and learn to identify their emotions more quickly when women "show them the way" (so to speak). This is especially evident when identifying the "softer" emotions. Men report to me that acknowledging these emotions makes them feel "weak" and vulnerable.
Hopefully, someone with a larger volume of experiences will respond more usefully!