Honestly, this poll is really too basic. The first thing that needs to be answered is, what type of corals are you keeping? How large is your tank? How deep and how long? These are very important factors to consider when deciding how to direct your flow.
For our 6x2.5' bare bottom prop tank where we keep mainly sps, we have two very strong powerheads on each end and face them towards the opposite ends of the tank. We also have our return that comes out a little left of the center and it puts out a lot of flow as well. I try to direct the flow so that it is somewhat circular. The softies I do have in there wave around as if I had a wavebox and I have it setup so all of my sps throughout the tank are getting high, indirect flow. We use penductors on most of our powerheads, these help small powerheads put out higher, indirect flow. Most corals/clams really don't like to be blasted with very direct flow.
The other reason we use lots of current through our tanks is to keep as much detritus in the water column as possible so it can be filtered out by our skimmer instead of it settling on the bottom or on corals.
For my softy/lps tank I have medium circular flow but nothing is getting very direct current because they don't need it or like it.
If you have a fine sandbed it is better to have your flow towards the middle/top so it doesn't blow your sand all around and irritate your corals. Usually the corals that prefer lower flow are the ones kept on the sandbed anyway. I think its good to have at least one powerhead to keep the surface of your water somewhat agitated, because it looks better and is good to airate your tank.
