It should be wherever you want it.
The advantage to having sand in the sump is that it helps to breed critters. It's a little easier to clean in the sump as you aren't disturbing the DT. Keeping it out of the DT means you don't have to worry about cleaning it. On the other hand, most people have more room, and hence more sand capacity in their DT, and the sand looks good. It also provides a home for many interesting critters in the DT where they can be seen more easily.
In my tank, I'll have sand in both (it's cycling now). However if I were running a store, I'd keep it only in the fuge and maybe one DT.