For me......Flow is often determined by the type of fish kept.
At the moment I keep riverine/ rheophillic species, so in order to best replicate a riverine habitat, I put overflows on one end, with a 1500gph influent (water entering from the sump) on the other end, so currents flow along the entire length of the tank.
I also use an 800gph powerhead to accentuate flow down the same length for more rheophillic species.
For lacustrine species, an overflow on each end might better imitate natural habitat.
For those species (tigerish, etc) that appreciate counter current, I might use 2 powerful powerheads aimed to clash to create that type zone, mid tank, along with multiple points of entry from .