I personally would use the 18 x 18 x 18 as the 12 x 12 x 18 really is only 11 gallons which IMO is a bit to small for them, the 18 x 18 x 18 is 25 gallons roughly, a far better size. I personally feel adults should be in 15 gallons or larger, its just an opinion though. And the nice thing is that because these guys are addicting, they are like chocolate (sorry I dont like potato chips), cant have just once piece! Going with the 18 x 18 x 18 will be large enough for 2 adult females if you happen to get so lucky as to get 2 of them!
But remember when you first get your crestie youre probably going to end up getting a juvie in which case you want to start with something much smaller like a kritter keeper, I normally go with the medium-lg size and they can stay in that until 8-10g then they can be moved up into a 10 gallon tank, the 12 x 12 x 18 eco terra untill they are around 15-18 grams then they can be moved up into something larger.
See to me that exo just seems to small. I think as a QT tank it is fine, but I feel they do better with a little more room. I am not saying they wont thrive in the smaller exo, but this is also one of those instances where each person has a preference, kinda people are with substrate, everyone will have their own opinion.
This is my 18 x 18 x 24 exo. When styling a tank you really want to have at 3 of the 4 sides of the tank covered and at least 50% of the cage to be decor
These are some of my other cages
Now thats not all of them, I have more on the opposite wall that is another juvie section
Wow, I love your setup! I need to get better shelves for mine. The ExoTerras are great but they're heavy, and they tend to make melamine sag after a while.