I always thought that Joan and Peggy represented the diverging image of women during the 60's... Joan thrived in the world where she used her sexual prowess to her full advantage. Peggy, being less gifted in the physical attributes department, was more into proving herself and letting her talent get her where she wanted to be. Remember, women's lib and bra-burning happened in the late 60's. Peggy was the epitome of the woman who didn't need a man to get ahead (and at the time many men did not appreciate such women).
Contrast this to Joan - I remember the episode where Joan was sooo good at reading copy for upcoming soap opera episodes and figuring out product placement and then having Harry hire a male into the permanent role. It was clear no one took Joan seriously. As Jim Hobart did not - "did you inherit your partnership?" No, it was worse, it was her using her sexual prowess to land the client and the shares...
I always thought the last cat fight between Joan and Peggy would be over this - Joan slept her way to the top to great financial rewards while Peggy worked her tail off for a mid-level job (remember when Joan, a partner, informed Peggy she would become something like Assistant Copy Director or some other meaningless title).
While they had achieved their status in the confines of SC&P, the real world hit them like a brick. Sure, Roger could be coy with Joan. They had been friends / lovers going way back. Peggy had made her mark with gaining respect from her co-workers. And then they go to the meeting at McCann... they are treated the same way as they were at the beginning of the show in the early days of Sterling Cooper... only now it is not okay. It showed how much both of them had grown.
And, at the end, we get confirmation of that... We were always teased with the idea of Joan needing to be taken care of by a successful man - Roger, Greg a potential doctor, and Richard, a rich retired developer. However, it was easy to overlook that Joan had always longed to be accepted for her talent as well as her looks. The script reading, the relationship with Avon and a few other clients. So, she seized upon the opportunity immediately when it presented itself at lunch with Ken at the expense of a cushy personal life that seemed to await. Of course, she twice asked if Richard was willing to marry her and as he never did ask, the answer apparently was no; hence, he was no better than Roger who saw her only in sexual terms and never took her too seriously.
Peggy always wanted acceptance both at work and in her personal life, although the latter wasn't always on display. She didn't back down when the account was taken away from her and called the other lady's bluff to get the account back (I think in an early season she let Pete or someone take an account and she didn't raise much resistance). And, she didn't sacrifice to win over a man. She is playing on her own terms both professionally and personally.
Well, done...
