I'm highly invested in Peggy's story, probably because in many ways, she is me, albeit twenty years earlier. There were ominous hints that things won't go well at McCann (such as Hobart saying he wasn't sure her status as copy chief would last long), but I'm hoping Weiner sees her as the only true winner of the bunch.
My take on Joan differs from what most in this thread have stated. She got to where she was by trading on her sexuality, and then when she tried to co-opt the women's movement, it failed as inauthentic. Of course, the whole environment in sales was total sexist b-s, and hasn't necessarily changed. If she hadn't been in such a rarified situation at SCDC, and had to work up from the bottom, however, she would not have been so thrown by what she encountered. By contrast, Peggy knows where she stands, and knows she's just using them to get her to the next level, so "screw them!"
That said, I can see the trajectory of a character like Joan as she marries the wealthy developer, moves to Cali, where she is able to be entrepreneurial and ends up the head of a wildly successful company.