as glod said depending on the species you have, and also wether you want plants ect.
sand take a little more looking after as the sand may well compact causing anaerobic areas (it`ll turn black)
so need stiring every time you do a water change, and also cleaning the poop out of the tank is harder as you have to hover the siphon over the sand but not touch it (its easy after you`ve done it a few tmes)
where as gravel is easyer to clean but just dosent look as nice in my opinion