As you are starting new with this 20g, I would also consider fish, as some have a substrate preference. Sand works best for the fish who like to browse over and into the substrate, such as corys, loaches. Plants grow well in sand or fine gravel. Darker rather than lighter is best for fish.
In a 20g you don't want more than 2 inches depth overall, meaning when spread evenly, for fine gravel; sand is probably better at 1.5 inches. You can build this up a bit more in the back if larger plants need it.