izigame.me

Disclaimer: I am only writing this in anger of reading comments like "The new planet looks pretty, but...". Yes. That is the actually the point of the game. So, for just a measly $5, here was the difference between the worlds for me:
Prime
The progression of the world feels very earthly. You melt sections of the map revealing new resources, until it is "terraformed" and plants/trees/animals begin popping up around the majority of the world. A majority of the land itself, the cave structures, the plants, etc. although alien and fun, resembles a lot of earth. There is, in my opinion, a very entertaining storyline to go along with that theme, but that didn't really feel like the idea behind Prime. You got to see this beautiful world transform before your eyes that did not exist before, and the end result was a very earth feeling world. And yes, there were some interesting things that happened to take place on the surface of Prime. But the idea is the planet.
Humble
Humble flipped the script a little. It is not Earthlike at all - that was the first thing me my friend said to each other when we opened it. And that ended up being the kicker of it: Humble is very unlike earth, in that its alive. Which, yes, this is kind of true of earth, but Humble takes it very literally. As you terraform it, the world is actually changing, growing, the land is no longer the same, the mountains have moved, things that you have mined off the surface came back, because the planet is now alive and regenerating. The caves you unlock reveal humongous mushrooms (which are now alive and growing again) and have been ripping apart the crust of the planet, creating a massive cavern. Inside the cavern, a singular tree unlike any other grew. One which was, by a root network inside the cave, connected to the entire surface of the planet, and as that tree grew and came back to life, you realize that tree is the reason the planet is completely different now. And then you look at the entire map, and you look at how the water that you melted out of the glaciers is flowing like blood into the center of the planet, to this one tree. And that, to me, is the lore mentioned in the description:
Why is this planet unoccupied? Once terraformed, it's "alive" again, shifting the terrain in ways that destroy everything. Its unpredictable, and the planet itself has managed to kill anything that stays on it too long.
Anyway, I had fun for $5