Much could depend of what kind of fish you have, while for most it doesn't matter.
But .....if the fish are riverine and live in nature in a directional flow, having the return at one end with flow running lengthwise pointed to the other end where the filter intake is works. Examples of rheophillic fish would be cichlids such as Theraps, Tomocichla or Chuco, ATF, or Dorado might be fish other than cichlids with the same preferences
Some of my fish are rheophillic, and appreciate a countercurrent zone, so I have flow hit against the normal length of tank, at input on one end, out the other.
This creates a small countercurrent zone at the flow entry end
you can see the way the plants bend, flow is opposite of the way the water hits the tank.
an example of directional flow may be the way this venturi valve sends flow along the length of this tank.