I believe it's best to have them both at one end, I feel that this way the water is going in a circular motion. Water is drawn in at the bottom and pushed away at the top. Makes the plumbing easier and neater too.
I have a 2026 in mine and don't use the spray bar, I have outlet going into an external heater then stepped down to 12mm pipe, this comes into the tank at the back corner slightly faced towards the side glass.
IMHO having them at different ends can mean that filtered water is pushed towards the intake.
I kind of thought the same thing so I sort of cheated. I put an HOB on the back left of the tank. Anything that gets buy the canisters intake gets caught by the HOB intake. The HOB also causes a small cross current that stops any dead spots under the spraybar. I know the water circulates throughout the entire tank because I can watch a leaf go around the entire tank.
I'd say that in along tank 48" and above you are going to have dead spots wherever you put your pipes, these are easily cured by using a small powerhead.
Other adavantages of having the pipes at one end are they are easier to hide, mine are behind an artificial log. Also draping the tubing over a long distance worries me, the weight of it could pull it off its connectors.
I dont have an Eheim but I setup my fluval so the intake was at the bottem right corner (not dead corner though) and the output was somewhat in the center left, I directed the output to the front left corner of the tank.
I'm pretty sure I am getting very good circulation, especialy since a 204 is really probably too much filter for a 20 gallon tank (water in the tank is cycled though the filter hundreds and hundreds of times a day.)
I don't see how you could get much better than opposite corners of the tank (Back Bottom Right and Front Top Left)
I dont use a spraybar though I'd rather have the current directed at the corner.
The filter is pretty far away from the tank I used all the tubing it came with, and it also has to pump the water up a about 3-4 feet but it works very well.
(water in the tank is cycled though the filter hundreds and hundreds of times a day.) 