Good things about sand: Don't have to gravel clean, eel like fish can hide in it, cichlids can move it more easily when digging pits to breed in, can suck off the waste just above the sand with a siphon.
Bad things about sand: Can't use an undergravel filter, good bacteria can't colinize easily, may get lots of debry floating around if filter isn't efficient, can't catch kuli loaches! :lol:
Good things about gravel: Can use undergravel filter plate to ensure bacteria are stable, traps floating unsightly waste, can remove debry very easily with gravel cleaner (siphon).
Bad things about gravel: some fish can damage themselves on course gravel, if undergravel filter is not used decaying food and waste can get trapped inbetween gravel and pollute the system, gravel cleaning can be a problem if not carried out regulary.
The choice is yours, good luck.