I think it's difficult to say what's best, but I personally observe tiny floating dust or detritus to see if there's sufficient flow and no dead spot. I can't technically measure the flow rate, but the dust flow gives you a better chance to tune it.
I personally use QP-16. Out of my experience with MP40 and Tunze 6095, I liked QP-16 the best. Moves a lot of water and the quietest. I also have one on each end, one near the back glass and the other near the front glass so that the flow is circular. I experimented many configuration, and figured circular flow is the best if you want to achieve nice flow rate. If the flows from both sides crashing at the center, a part of the flow basically returns back to the same side, meaning such a flow is no longer fresh (this is just my guess, though!). Acros on the side has to be exposed to the same flow that they have once consumed a bit. But with circular flow, when it turns back to the same place, some portion is gone to the sump and it's mixed with fresh returning water from the sump. You can also observe that there is no stagnant protein on the water surface if your water movement is circular.
I'm not a chemist or biologist and I am not certain if my assumption is right. And, like many says, there is no such thing as a perfect answer. Many people are probably successful with many different ways. But my acros look like they like it.
Yas
