It's a tough call. We've kind of grown up with Ellie, and seen both her at her best, and at her worst. And while the course of her journey is understandable, it's rough to watch and have her back through and through.
Meanwhile, we spend less time with Abby. She's self-admittedly a rather terrible person, and we see her make morally questionable at best choices. However, again her journey is another understandable one.
However, of the two, we saw Abby come back and try to do better by others. We saw Ellie pull back at the very end, but we haven't seen her demonstrate anything beyond that. Aside from realizing/confronting what Joel would have wanted from her in that moment, we don't know anything beyond that she's feeling the loss of her family and her ability to play a guitar. And in some ways I feel like that's where a Part 3 comes in. What it means to live and give to the world, even if it's not the way that Ellie had imagined when she set off with Joel and Tess all those years ago.
So I'll go with both, even if time gives Ellie an edge.