We use Kraft Indented Paper for our adult and sub adult boas and paper towels for babies. The Kraft Indented Paper gives the snake some textured flooring, it's pretty absorbent and looks better than newspaper in a cage. Aspen is ok for tubs, but in cages it can be a pain to maintain really clean. We prefer to clean the entire surface and replace the substrate when a snake urates or defecates Vs cleaning a section.
Just my $0.02.