Capped or uncapped substrate

My MIL has a 3 gallon for her Betta, and I've finally convinced her to get a bigger tank -- likely a 5 or 7 gallon long. I'm case, my question:

Should I put sand over the dirt? I've seen some people here say no, and I'm curious to know what will be best for a tank that I hope will grow fast and thick, as she isn't the best at maintenance.