A rather late contribution, but it may be helpful to someone. It will be interesting to see the specialists' reaction to a simple civil engineer's opinion.
Don't forget the option of mechanical variation. They can be supplied to set up manually, or can have a little servo to provide logic controlled speed variation.
One of the advantages I see in this method is that you will definitely buy the whole package from the pump supplier and won't have interface issues between sources if there is a performance problem. Pump suppliers are fully competent and working on home territory in the area of the mechanical gear box, and motor selection is no more critical than the FSD case.
In contrast, you Variable Frequency Drive would require supply of very specific equipment from at least two sources, and possibly more: pump manufacturer, VFD manufacturer, Panel builder and, possibly, motor manufacturer to allow for VFD requirements. Your pump-man may claim to be happy to do the full package, but he is only doing it by subbing out and therefore putting more levels of interface between the buyer and the true source.
It does depend on the application and even I have accepted VFD for large cenrifugal pumps for potable water distribution, but such things as sludge pumps around a sewage treatment works may well be candidates for a mechanical solution.
As ever, there is no universal solution and if all goes well, thats fine. Unfortunately, my experience of problems arising where there is an extended contractual string has been to discover row upon row of perfectly matched sloping shoulders. And if there is any IT involvement... Oh don't let me start on that - the web-site authority would have to bar my access!