From a developer standpoint, when it comes to a new runway (o'hare) vs a new terminal (houston). Is it much easier to update an airport like o'hare then it would be for houson?
You can't really say, it depends how the new terminal is, if it's a completely new build with a completely different style and materials than the others, it will require more work than adding a runway. And, even if it's new, it still depends how it's made and how complex it is from a modeling point of view.
Adding the runway at KORD was a big job, because about 30% of the whole background had to be remade and in P3D had to be remade in 4 seasons. Change of taxiways required to change lots of taxiway signs and markings, and custom 3d lights too.
So no, you can't say which one is more complex as a general rule. It all depends.
