There are certainly more qualified people than me to comment on caging requirements for arboreal boids. However, I've been around this hobby a long time and have noticed a shift in arboreal keeping towards cages that have more lateral vs. vertical space.
And from my minimal experience with both species I agree completely. I believe that this has been discussed on the ETB site on kingsnake's feature page as well as on Signal Herpetocultures site.
You might go to both of those places to read some more.