In my humble opinion
Paper towel --- one ply is too thin in case the snake spill the water when it battle with the salamander but too many ply will let the worms/salamanders get under it. It happens to me that the salamander will splash a lot of water from the water bowl when the snake try to pursuing it. The need to change the paper towel too often is troublesome.
Aspen --- ??? Is that some kind of tree leaves, never try that before and I dont want to let the snake hiding just anywhere it want.
Newspaper --- I worry the lead in the print may harm the snake in the long run and it don't look natural.
Soil --- prone to bacteria and fungus build up. We never able to create the perfect ecosystem in the tank and the bacteria will keep growing without control because of the moisture.
Repti cage carpet --- good choice with different color to choose from but I hate the cleaning job. The color of the carpet is quite appealing.
Disposable cage liners --- I have no experience and still need some opinion.