Ive had both and i like the classic, natural looking gravel. The sand i had ALWAYS got stuck in my filter and I frequently had to go and clean the entire filter to try and find the grain of sand that got stuck.
It was very frustrating. I also know a TON of people that wont touch gravel...sand is all they use. Personal preference!
Sand. fish seem happier and its alot easier to take care of. You can vac a gravel tank today and its dirt and vac it tomorrow and it seems just as dirty. With sand seems like more of the fish poo ends up in the filter then on the bottom.