The sand will eventually compact , creating anerobic spots which are bad for plant roots , & toxic gas pockets will eventually form . To get around this , you will have to occasionally disturb the sand throughout the tank to prevent this compacting .
On the plus side , it's much easier to vacumn all the waste off the surface of the sand than it is with gravel .
Overall , I'd say that a sand substrate will require more maintenance than a gravel one . But sand does look pretty cool .