i believe a mix of all is what your looking for, you want surface aggitation, but you want enough current deep, so it stirs up detrius and keeps it suspended untill it gets skimmed out. AND you want good flow across your corals that like good flow.
yes, both. Surface agitation aids gas exchange which is very important. Adding flow along the bottom helps to keep the detritus in suspension so the skimmer can get it and pull it out of the tank. if your using power heads then you have the opportunity to play with the flow directions and see what works best. The surface part is easy just have one pointed along the surface to create some good movement. For flow in the lower part you will want to try a few different things. try each position for a week or so and see how much stuff builds up on the bottom of the tank. When you find a position that helps keep the bottom clean then there ya go! Its best to also have the rock stacked in a way that allows some of the flow to passthrough the opening too. And also its a very good idea to have the rock on risers so its not in the sand. use pieces of PVC cut a little longer than the sand is deep so the rock doesnt contact the sand. Kinda like stilts or platform shoes! This will also allow some flow under the rock and prevent some build up there too.
If the sand is new then you may need to wait for it to get a good bio film on it so it is less likely to blow around. Once thats dont it should stay relatively secure allowing for good flow on the bottom!