At that HP and that application, it’s almosr a toss-up. But there IS a difference.
The 750 series is an “Architecture” Class drive, they are designed with the end user in mind, so they are designed to last. The components used are selected for life expectancy and they are repairable in the future. The 755 has a lot more performance capability, but those are probably moot on a centrifugal pump application. One thing it does have however that the others don’t is what’s called “DeviceLogix” programming capability, essentially a small PLC embedded into the drive to where you can run programs and control I/O in the VFD application or even separately. That’s one thing you should double check, because the 520 drives don’t have that feature.
The 520 series drives are “Component Class” drives meaning they are parts and are essentially “consumeable” in that if something happens, you replace it. They are mostly targeted for volume OEM sales to be competitive with other similar drives, so component selection is shifted toward being lower cost and they are not designed to be repaired in the future. The one caveat however is that the 520 series has two modules, power and control, so it is “partially” repairable in that if one or the other module fails someday, you only need to replace that half of the drive. The other general difference is that with Rockwell, all Architecture drives have a DC bus choke, which comes with some long term life expectancy benefits, whereas Component class drives typically do not. HOWEVER, in this specific case, a 15HP and up PF525 does happen to have a DC bus choke. Hence the toss-up decision in this specific case.