In a low restriction loop, a D5 can put out more flow than a DDC-2 with top. The DDC-2 however has a much higher head pressure rating, and hence in a more restrictive loop will outperform the D5. However, both pumps are likely to net you quite a bit more than 1gpm per minute in most setups, so even if you use a D5 in a low restriction loop, you would see no temp benefit over a DDC-2 or a DDC with a top. Since the DDC is so quiet, I chose it for my loop as I am running the same parts as he is, and I have seen no difference between it and the DDC-2 that I was running in this same loop, both using a top.