[ponokareefer]

Optimal sump turnover is between 3-5 times total system volume per hour. On a 125 gallon system with a 50 gallon sump, that is 175 gallons of water, minus rock, sand and corals. You typically take out 10-20% for rock, sand and corals, depending on how much you put in your system. At 10% off 175 gallons (158 gallons), you should be aiming for between 474 gallons/hour to 790 gallons per hour.
The mag 9.5 is rated at 975 gallons per hour pumping 0 feet up, 780 gallons per house at 1 foot, 740 gallons per hour at 3 feet, 685 gallons per hour at 5 feet, and 610 gallons per hour at 7 feet, and 450 gallons per hour at 10 feet. You are in a good range if you are pumping anywhere from 1 foot to 7 feet above it.
The mag 18 pumps at 1500 gallons per hour at 9 feet above it and still 875 pumping it 10 feet above it. Unless you are pumping the water over 10 feet above the sump, or have a tonne of elbows in your plumbing, the mag 18 will be too much pump.