what type of substrate is best for a QT if any

tank is 20 gallon
filtration will be x2 whisper 30's
heater a 150 watt with thermostats on each end to control all over temp
just hiding places and maybe some driftwood for BB growth
setting it up today.

i have about 18 pound of sand left over from the other tank so i was going to use that but if it is more ideal to use gravel im all for it
as always thanks in advance