There shouldn't be a big difference in humidity- the humidity is mostly controlled by the amount of air-flow (amount of ventilation) combined with your choice of substrate (some hold humidity much better than others). Personally I'd go with 18" wide- because 12" is vary narrow for a wide-bodied snake to turn around in, especially when you have hides (2- one cool side & one warm side) plus a water bowl in there too, at the very least. (Many people add other decor too.)
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