I like Keiffer more, but Ferris hasn't fully developed as a character yet, so I don't have a complete view on her.
Ultimately, I think Keiffer's interest in Ryner was a schoolgirl crush that sort of blossomed into something a bit more intense, as she was having legitimate fears of Ryner getting killed during her betrayal, something I wouldn't imagine if she prioritized her sister above everything else. Then came the prison scene, which completed her development and sealed her love for him. He knew and accepted what she did, and yet still gave up his freedom for her - it added a deeper attraction to the superficial crush she had earlier, leading to full-out love.
I don't think Milk full-out loves Ryner, it seems a lot more superficial than Kieffer's.
And Ferris pretty much started out as a child when she began her travels with Ryner. Even now, I don't see her as a mature women, but a young girl. She didn't even know what love was when her journey started, and her whole dango fixation is really juvenile, not a whole lot different from her younger sister. But over time, the fixation has mellowed out, and her attatchment to Ryner has increased. I really liked how when Ryner went berserk, she didn't say something cliche (and wrong) that would betray the development. She called him an important partner, which was what he was!
She hasn't understood love yet, so it's a different, longer development path from Kieffer. I'll only really be able to compare with fidelity once her development is done, but for now, I like Kieffer's better, because I like the whole redemption theme going on with her. Also the fact that she's more "normal" than Ferris, and isn't connected to a house of power and influence in Roland.