HIs are a bit more "arboreal" than other BCIs probably because they are lighter bodied and shorter. My cages are 4x2x2 and have branches and/or "jungle gyms" that the snakes use quite frequently. With that being said... It really is up to you what you get. I dont think that the snake would really care either way. If you are planning on a large collection and space is a real concern then the 12" tall would be the way to go. If you have the room to spare an 18" or 24" tall would be great.
Do you have any pics of your HIs cage?
Ball Python, Bredl's Carpet Python, Kenyan Sand Boa, Saharan Sand Boa
Mexican Black King, California King, Snow Corn, Okeetee Corn, Everglades Ratsnake
Blue Tongued Skinks: Irian Jaya (2), Indonesian, Northern, Tanimbar, Eastern, Kei, Merauke
Crested Gecko