You will get a lot of opinions...try using the search function on the site too as this discussion has been had a few times recently.
Personally I really like the look of sand, initially its cheaper than gravel you'd get at the store assuming you get playsand, but its pretty labor intensive to get it washed and into the tank...then you need to keep up with tank maintenance because fish waste will be more obvious with sand than it is with gravel. A lot of fish really like the sand as opposed to the gravel also. Sand isn't that great if you want a planted tank, as well you really can't use an undergravel filter with sand.
Gravel seems to be a lot easier to clean, easier to deal with in general. Lots of different sizes, colors, types...
Overall I can't say I have a real preference one over the other in general...but if you answer Hyunelan's questions it might help us help you decide which would be best for you?