I always answer this with that a person purchasing the controller needs to look at the feature of both GHL, and Neptune Systems and decided what features they want and the most important to them and then decide.
For me GHL, stability was a major factor for me, and 4 years in June since my Switch from APEX 2016 to a GHL P4 I have NO regrets doing it. Fast forward to today I would hand down do it again no questions asked.
I see my controller as set and forget device you should not need to worry about what the controller is doing and have confidence it is doing its job. It like you getting into your car do you worry about the onboard computer that runs your car, you put the key in or push the button and it starts.
Before my switch out having the Apex was like a job always an issue, the salinity probe was always not working right was always in fusion adjusting something, it was taking the fun away from having a tank. I am away sometimes away week on end and never had to worry about the unit failing even when power is out and the standby generator running everything, I cannot say the same for the Apex 2016.
I have helped user switch from Neptune Systems to GHL, even answered with Apex Fusion screen shots how it done in Apex Fusion to GHL Connect. Apex can do things GHL can not do and GHL can do things Apex can not do. People are loyal to both manufactures. But I believe GHL can not be beat in stability, and reliability.