I'd go with sand but if you are going to truly have a FO tank then I'd just set up a shallow sand bed (unless you are planning on the substrate providing a high level of filtration) .....
IF you have a lot of other filtration then the substrate will be a finishing touch and not an integral part of the tank. If you are planning on eventually moving it to a reef tank or think you might want one day do a reef then I'd start with a deep sand bed - 4-6" or so ....
HTH