I think it depends on what you want your WB decks to do. Sorin is probably the way to go if you have any kind of tokens subtheme, or if your white and black section has lots of ways to use tokens (white lords, black sacrifice, for instance).
Kaya meanwhile functions as a more general win condition in a midrange or a control deck, but you need to protect her. She can help stabilize your life total against aggressive decks or get you some card advantage over a few turns. She's not really powerful enough to warrant splashing her in my experience (nor is INN Sorin), but is a fun enough card in an orzhov deck. She notably has some blink synergies, too, which can be relevant with a lot of white cards, black not so much in my experience.
This card seems so strong that I'm not sure I want to cube it. Imagine an aggro mirror where this card is cast on turn 3. Seems like the end of the game more or less! And that's just the 2 damage mode. Casting this off of Laelia, the Blade Reforged, Light Up the Stage, or Expressive Iteration is just absurd.
I think the function of this card is just a little too awkward for me to cube it. It's cool that you can target your own lands to essentially ramp, but I'm not sure how needed that is in a reanimator deck, or especially if you're hard casting this card. Copying a planeswalker and only ending up with a single 3/3 feels odd as a play pattern. That's probably pretty strong alongside a 4/4 flier, but it feels bad to lose out on the second token.
Part of what makes Terastodon so great is that it is flexible, and that there are scenarios where destroying 3 of your opponent's lands essentially seals out the game. Whereas with this guy, you're just as vulnerable to a wrath as before he enters the battlefield.
As for big blue creatures, I think Consecrated Sphinx, All-Seeing Arbiter, etc. still take the slot as they are a lot more castable in a blue control deck.
Kaya meanwhile functions as a more general win condition in a midrange or a control deck, but you need to protect her. She can help stabilize your life total against aggressive decks or get you some card advantage over a few turns. She's not really powerful enough to warrant splashing her in my experience (nor is INN Sorin), but is a fun enough card in an orzhov deck. She notably has some blink synergies, too, which can be relevant with a lot of white cards, black not so much in my experience.
Part of what makes Terastodon so great is that it is flexible, and that there are scenarios where destroying 3 of your opponent's lands essentially seals out the game. Whereas with this guy, you're just as vulnerable to a wrath as before he enters the battlefield.
As for big blue creatures, I think Consecrated Sphinx, All-Seeing Arbiter, etc. still take the slot as they are a lot more castable in a blue control deck.