Your new betta is beautiful, fiery DT! Very nice. :D
Less substrate = less area for toxins to build up, and easier to clean.
I've literally used EVERYTHING, and nothing (best for hospital tanks).
For my main tank, I've finally settled down to glass 'gems' (round pieces of glass, about 3/4"), just enough to cover the bottom of the tank. They are pretty and easy to clean. They are too heavy for a tank vacuum to pick up, and are easy to clean - the crud is easily sucked up around them. No toxins can build up beneath them, if used as I described. Thick layers of sand and gravels can become 'poison pits' (possible exception: use of under-gravel filtration).
I have two catfish. Glass gems are not the optimum substrate for them (fine sand is). The glass gems are too big for the catfish to sift, or move about like fine gravel. They are very smooth however, so the catfishes' barbels don't suffer.
If your tank set up is very small (under two gallons), use only a few glass gems for easy cleaning. Or forgo substrate altogether.