I have had sand, gravel and a mix of both (gravel on the sides, sand in an avenue up the centre). They both have their pros and cons.
Gravel is good in that you can just dig the siphon right in and suck all the dirt out. It holds most plants in quite well and, because it doesn't compact, is unlikely to form anaerobic pockets, especially if you are using a gravel siphon.
Sand is prettier if you keep it clean (in my humble opinion). However, dirt does show up on it a lot more than gravel, especially if you get a fine white sand like I have. To clean, you hover the gravel siphon a little way above the sand, sucking up the dirt without the sand. A turkey baster also works. You do have to stir sand every week to prevent anaerobic pockets forming, unless you have burrowing critters like Malaysian Trumpet Snails. It also holds plants in well, but without burrowing creatures can be too compact for plants to form complex root systems. It is great for cories and kuhlis, and my shrimp seem to love it too. Also, there are tales of piggy bettas swallowing sand along with food that has falling down, having it compact in the digestive tract and dying as a result. However, this kind of thing is a rare exception, not a rule - I have kept 8 bettas with sand and not had this problem.
Really, it depends how much work you want to put into it and what you prefer the look of. :)