Orchid bark every time for me ... I use it in all my 17 vivs .
Looks amazing , shows up the colours and patterns of all the snakes , holds humidity if you require it but can be used for dry conditions equally as well .
Plus it's re-usable if you are on a tight budget . I put loads of old orchid bark in a big tub , pick out all the obvious bits of crap and whatever , then give it a good wash in diluted F10 , then drain off and let it dry naturally or even in the oven if you want it sooner .