i have had both, sand looks better in my opinion but can be harder to clean for someone new to the hobby. Sand also provides a great surface for plec and cory's and loaches as it doesnt harm the barbels etc...
Gravel looks ok, but in my opinon can look a bit unnatural, however, it is easy to maintain and clean with a gravel filter. With sand you just have to create little circle motions above the sand to free up the dirt/etc..., with gravel you can be as tough with the vaccumming as you like.
I would look through a few threads with pictures and decide what sort of look your going for....sand can be very cheap if you buy the play sand from argos (just make sure you wash it first)
