@yates7592 - Here are differences I know between Reference 4 systemwide and True-Fi apps.
True-fi only uses a minimum phase filter (no pre-ringing but has phase shift)
Reference 4 allows selection between minimum phase, linear phase, and zero latency. I'm not sure of the filter method in zero latency. The linear phase filter has pre-ringing but no phase shift. pre-ringing is not audible to me personally. I use linear phase which matches the filter method I have on my DAC. When I used the default minimum phase fast roll-off filter on my DAC I would get a headache from the phase shifts, so I use linear filtering as much as possible YMMV.
True-fi is designed to be simple and easy to use.
Reference 4 systemwide has more options and thus is a little more complex but still user friendly IMO.
True-fi allow the bass to be tilted up or down and the treble to be tiled up or down after the calibration profile is applied.
Reference 4 allows for more modifications after the calibration profile is applied; such as:
0-100% "wet" (I think this means % of calibration applied)
Tilt - The frequency response can be linearly tilted toward bass or treble, I like this function a lot
Bass boost - Just the bass can be increased or decreased
headphone or speaker simulation - other headphones and speakers can be simulated
Reference 4 includes a plug-in version but I have not used it. Also note my experience with the True-fi app is from a few months ago. Sonarworks may have added a few new features since then. I am currently a happy Reference 4 Systemwide user.