Huh. Yeah, I just went back to look at Episode 10, and Madoka's witchout at the end of Loop 2 is totally weird. Her Soul Gem just kind of changes into a Grief Seed in her hand between shots, rather than dramatically shattering out of the Gem like Sayaka's did. Then she holds up the Seed (somehow still controlling her human body! WTF!) and it dents inward like there's a vacuum inside and black smoke starts leaking out to become her witchform.
I guess that even by that point, Madoka's potentially was so high that all the usual rules went out the window.
It's my belief that the difference has to do with the girls, not their potential.
Sayaka gives in to her sadness and curses the world when she declares herself a fool. Remember, moments before, she had either murdered or nearly murdered (depending on whether we're going by the manga or anime) two humans out of sheer hatred and grief. Then, rather than asking Kyouko for a Grief Seed, she just...talks to her, before Witching Out. That Sayaka had already become a Witch in all but form: A Magical Girl that has abandoned Hope and begun Cursing the world. She smiles brokenly just before Witching, and her Grief Seed opens up to release Octavia.
When she finally Witched Out, she did so
willingly. She
chose to forsake the world.
Madoka, on the other hand, lies on the ground screaming in pain, snarling and struggling. She doesn't cleanse because
she is unable to, not because she no longer cares. She holds her Grief Seed up, and it
refuses to open. Instead, it sits there in her palm...and Kriemheld Gretchen
rips her way out. Her last sound is not a word, or even a sob, but a shriek of unimaginable pain as this happens.
Madoka did not give in to her Grief. She did not
choose to forsake the world. She Witched Out when she physically could not hold on to Hope any longer, and her Grief literally overwhelmed her. She refused to Curse the world with every single fiber of her being...but still her Grief broke free.
It's one of the reasons I admire Madoka so much: She held on when no one else did, fighting without hesitation, never surrendering...only being overcome. As someone who's faced his share of despair...that's absolutely amazing. It's a miraculous kind of resolve.