I'm quite liking a mix of cocofiber (75%), vermiculite (15%), and mixed mosses (10%) for my Ts. True spiders that are local to me get 40/40 potting soil and vermiculite, plus 20% "true dirt" including sticks, leaves and moss that's literally just a handful or two that I grab from whatever area is nearest to the spider. I do put this in the oven and sort through thoroughly before using, just in case there are little bugs or anything else unwanted like a dropped bit of rubbish or broken glass.
I use 75% cocofiber substrate and 25% peat moss. Have tried using vermiculite- it retains moisture a bit better, but doesn't look as pretty in my opinion. I've never had a problem with mold growing on the cocofiber as some people mentioned.