Here's how it went for me:
In season 1, for the first few episodes, I was kind of tied. I thought the show did a nice job of setting up the talented B'way vet. who has been paying her dues, and needs her big break vs. the "Peggy Sawyer" fresh faced innocent. There were reasons to root for both of them. Ivy was more Marilyn while Karen was more Norma Jean. (Honestly for a while I thought they were going to end up splitting the part and have one girl play Marilyn, and the other play Norma Jean with a full on duet LOL.)
Then Ivy started all her antics. Karen was in the chorus, learning, working hard, and Ivy was being a total brat to her - and lots of other people - not to mention having huge meltdowns - not to mention that she (like Daisy in season 2) slept with the director to get the part, and Karen refused to. Then she slept with Karen's fiance! By the middle of season 1 I was TOTALLY team Karen. Karen just seemed to deserve it more.
But that full on changed at the start of season 2. See, what I think Karen had going for her was that Peggy Sawyer quality. Well, after she became Marilyn she was no longer Peggy Sawyer. She started to turn into (almost) everything I didn't like about Ivy. Now she was the star who was being a brat to everyone, treated like God's gift to the universe, and acting totally entitled. There was no humility, no "wow, I got my big chance, I'd better not screw it up." She was now treating other people the way Ivy had treated her in season 1. Suddenly she went from young innocent we were rooting for to bratty young girl who got everything WAY too easily and never actually had to pay her dues.
And then Ivy calmed WAY down. Seeing how fast Karen turned into a brat I think gave us far more sympathy to Ivy. Ivy'd been at this a long time. She HAD paid her dues. And at the end of the day, despite the fact that she was far more prepared to play Marilyn then Karen was, Derek chose Karen because of some etherial "muse" quality. Not fair. Then Ivy started getting a chance to prove herself away from "Bombshell." She basically saved "Liaisons" with her awesome performance, and diplomatic skills. (Well, at least made it somewhat able to sit through.) She had learned her lesson, and was working hard.
Then entitled Karen gets a crush on some guy and throws away everything she had been working towards - and the biggest break EVER! To follow him to the fringe. What? Even though he'd been nothing but a total jerk to her.
And then Ivy finally gets her big break. And she's more humble this time. In reality, she's FAR more right for Marilyn then Karen was. She has to deal with an insane amount of drama, not to mention the shadow of how "great" Karen was in Boston. And finally, she gets her due. She stands up to Derek, and basically gets her life back together.
So that's why I ended the series as a MAJOR Team Ivy fan.
Madame Morrible: "So you take the chicken, now it must be a white chicken. The corpse can be any color. And that is the spell for lost luggage!" - The Yellow Brick Road Not Taken