Think of the flow as being like a mushroom cloud, it domes and bounces back when it hits the far end, but this won't happen on a tank that is so long. Conversely if a tank is very narrow, it focuses the flow so it reaches further. In a really large tank without much constraint to width or length the flow should cover about 1.25m in width and 2.25m in length. As for what would be a better, if the tank is SPS, then I think the 6508 would be a better choice, if it was primarily fish and soft corals, the 6255 should do the job, you could always add more if needed.