I have a Mag 7 on my AquaC 120. Seems to be about the right pump. It is nearly plug and play, with very little adjsuting needed. IIRC, they recommend the Mag 5 for the 120, and it was underpowered IMO. They also used to recommend the Rio2100. It worked better with the Rio 2500. But I've had several Rios fail on me over the years, so I prefer the Mag pumps.
In short, go with a Mag 7.