A Matter of Clouds

Hey guys, here are some environment design sketches I made for the first part of Gaëlle Séguillon's class on Learnsquared.
I wanted to try some cloudy environments featuring Sci-fi Structures.
First 2 pieces are a bit more polished and based on sketches, as 2 last are a bit more rough and quick and started from pictures.
Which one do you guys prefer?
Cheers