Its kind of a balancing act with substrates, heres a basic rundown...

Bark- Looks natural, can be misted w/o molding. Keeps humidity but dries out fast. Kind of hard to pick out feces and urates though sometimes though.

Moss- keeps humidity, but is damp so chances of your snake getting belly rot is very high. Wouldn't recommend it but maybe in small amounts, inside a hide, during shed times. I knew a guy who kept a bp on moss and it actually died from a bad case of belly rot. It was pretty neglected overall though. Theres better ways to get humidity levels up than to use moss.

Carpet- easy to clean, holds no humidity.

Aspen shavings- never had any personal experience with it, but I've heard good things about it. Remember not to let it get too damp, mold and bacteria love warm moist surfaces.