what's nice about sand is that when your ps are going crazy, it will lift up a bit and add a certain frenzy mood look to your hunt scenes. the bad side of it is that if sand lifts up too much, it will go in your filter and can do some damage. thats the reason why my local supplier gave me, its not based on prsonnal experience.he siggested me to put small gravel instead, which i did. it lifts a lil but not enough to go in your filter.
also, dark colors will make your ps look darker (which is considered to be good looking for a P). still, its a personnal choice.