The only substrate I've used is cypress mulch and that was for my Brazilian Rainbow Boa who needed really high humidity and was living in an Exo-Terra like yours. My BPs just sit on indented kraft paper either in my T8 or rack where humidity is easier to keep.

I liked cypress mulch because you could spray it down and it would hold humidity well, but could go a long time without getting moldy. The coconut husk probably is similar and I've never used aspen, but it seems like it would hold less humidity than the other two.