Which ever you prefer, Dirt has more nutrients for plants but can get very messy. Sand doesnt have many nutrients at all but you can use substrate fertilizer and isn't too messy. Dirt might be a bit more expensive (imo i'd get organic soil). You can get 50pound bags of play sand from homedepot for cheap.
Iv tryed dirt with sand cap,substrate,sand and seen close to no change in plant growth imo its just preference.I did mess around with diff lights and found that a 10 dollar wallmart plant light works extremely well for some reason.There are so many diff plant nutrient fluids u can add to your tank now a days witch makes growing damn near impossible to mess up.But i think about 2 1/2-3 1/2 inches of substrate looks awsome in a planted tank.
Dirt is not really needed for a successful plant tank. I prefer just clean sand or gravel. If you decide to use some lay it on the very bottom under the sand or your going to have a big cloudy mess everytime you disturb the sand planting or arranging.