MY OPINION i think you should go with the 20 long one is a simple but nice setup doesn't cost that much to keep it going an it may last for more then 2 years depending on the snake. i got it an I've had my bp in a 20 gall for a while they don't grow as quick as u think they will. so it would last for a long time an work great an as for humidity the light over the water bowl should help alot if u still have that problem u can buy little things for the humidity. but on the other hand if your getting a female
id go with the bigger one if u don't wanna have to buy another cage because they're way bigger adults then males but its still your choice