I started out using Forest Floor (cypress mulch) but switched to ReptiChip (coconut chip) because of personal concerns around the environmental impact of harvesting cypress trees. Both have been very good substrates - though I would give the edge to ReptiChip. It holds a steady humidity level very well, for various humidity ranges. I have had no problems with mold even when I use it in the enclosure for my young Brazilian Rainbow Boa, who requires 99% humidity at all times.

I use aspen alone for my Kingsnake and for my Children's and Stimson's Pythons. The Ball Python, Rainbow Boa, Green Tree Python, and Jungle Carpet Python all have ReptChip. Everybody requires somewhat different humidity gradients, and ReptiChip has been great in all cases. Once I run out of my current batch of aspen I will probably switch everyone over to ReptiChip, because it's less dusty, easier to spot clean, and seems to be great for everything from low to high humidity.