Crested Gecko HELP!?

I will be getting a crested gecko very soon, and I need to know should I get the 12x12x18 (12 gallons) Exo Terra Rainforest Kit or the 18x18x24 (33 gallons) Exo Terra Rainforest Kit? I don't have a ton of money so I would prefer the smaller one, but if it is really not big enough then I'll get the bigger, and I could always upgrade in a while... So what do you think?