I used to use aspen and ive switched to newspaper.
I have around 20 snakes so its easy and inexpensive to use.
The pro on aspen is its very absorbant and snakes can burrow it in if they like to. The con on aspen is that when i feed, sometimes im fighting to get aspen out of snakes mouths. I dont like risking them injesting aspen.
I like newspaper for the low cost, and ease to clean. They can hide under the newspaper if they like and theres no risk of impaction. It is not so good at holding humidity, if you have a problem with that anyways.