Cypress mulch is popular. I think it is more popular than coir & sand now (many don't use the sand any more too). No substrate is the "best". Its all about what works well for you and your tort and meets the humidity needs of the species.
I just have soil with hay mixed in. Dixie loves it. She digs her hide out every night (i fill it in every morning!) and borrows all through the tort table.
Murdoc I used to do coir/sand at first, and I hated it. Esp. How much of it ended up in their food and water. I use plain cypress mulch from lowe's now and I love it so much more. Plus it's easier to keep the moisture for my little babies.
We use 100% pure fir bark in what they call a small chip size here. You can probably get it at your local plant nursery..Edger seems loves it and it is easy for him to dig in.
oh haha i just saw this!! i happened to be at wal mart the other day and they were clearancing their cypress mulch for $1. i origionally bought 15, but i had to go back and only get 10 because no more would fit in my car! so, there's my substrate for the year! if you look i bet a lot of stores are clearancing out most of their gardening stuff.