I use a Neptune Apex Classic as the main hub and use a GHL Profilux Doser 2.1 so I could try both worlds before committing to one in the future on a bigger tank. From my perspective:
Apex Pros:
Setting up Fusion was exceptionally easy, and programming it took a little bit of thought but is fairly intuitive with the "if-then" programming style. I actually appreciate this more than just preset options (which they have, something as simple as "on -8am off 10pm". The email alerts were pretty quick to get set up, and overall the app experience is smooth and loads quickly. I can set maintenance mode functions based on what I am doing and access it all via the screen and touchpad or my phone/computer/anything. Another Pro of Apex is market support: they have several users and tons of troubleshooting out there, as well as tons of products that can integrate very easily into the system, and some are relatively inexpensive and can be found used easily on forums allowing more people to get into their system. That and their standard powerbars come with 8 plugs, getting you more plug per dollar. I also like how easy it is to integrate the break-out box and control many things based on simple open/closed switching making it endless for those who have the ingenuity and the time. Otherwise there are modules to do just about everything. Tons of youtube videos and other tutorials to help you troubleshoot, diagnose, and better understand and use your Apex.
Apex Cons: once you get beyond just the controller, powerbar, and regular probes it can become a bit of a wire noodle mess if you aren't careful and spend some time. Now this may not seem like much to others, but when you run several modules and equipment requiring plug control/programming, it can get out of hand quickly. To add more temperature probes, you need additional modules, etc. Now most of us likely don't run several PM2s and all of that, but some folks need an entire cabinet next to their tanks to house PM2, WXM, wifi adapter (for us classics/Juniors) which can be cumbersome and look like a fire hazard. Further, Apex I believe uses 24v USB for the aquabus which when running long distances can be hard to find the correct cable.
GHL Pros: complete integration of the controller into one unit, and add-on units are not function-specific meaning you don't necessarily need a specific module to expand assuming you have a free port. Once programmed, they perform as instructed and have some great features regarding alerts, alarms level indicators, stirring capabilities, and all of that (just within a doser!). I prefer the GHL Control Center interface, but I also appreciate that a simple web browser is achieving the same goal for the Apex, and the simplified approach the Apex takes to achieve similar goals. My doser pumps came out of the box within 1ml of the pre-programmed calibration rate and calibration was very easy to achieve. Having manual toggles for each of the pumps right on the face of the unit is also nice, and I appreciate the logo being used both as an alert and a status indicator since it is large and bright.
GHL Cons: documentation is ok to get started but not nearly as much existing support out there compared to Apex. They just launched a Youtube series and their email responses are always fantastic, but a google search for an issue might not always yield you the information you need like an Apex search would. I had a harder time getting the GHL onto the internet and then enabling myGHL, and the manuals in the box and their website were less than helpful until I came across a forum post where a user posted a much more in-depth manual so maybe this one is on me at first, but again stating my experience. The myGHL app is a bit slower than Apex and I find when using the GHL Control Center program on my computer over wireless that if I am part-way through tweaking a setting and the connection is lost, I lose any progress up until the last point I "saved" settings to the doser. I mention the GHL Control Center as it is the foundation for all of their products on the PC platform, and functions the same regardless of device (arguably a pro, but I imagine I would have the same challenges with it until I got familiar with it). Programming through the Control Center makes more sense the longer you do it, but at first I had a hard time with the translation from German to English and I back to my first point there isn't as much out there compared to Apex to help troubleshoot, so the less-than-savvy computer folks may struggle with GHL more than Apex to accomplish the same goals.
After all of this, I think both of them are great systems and I am currently satisfied with both. In the future I will likely try the P4, but I will sorely miss the 8-plug powerbars and second-hand price points of Apex! They can achieve nearly the same goals for all of us hobbyists and having both in this space will only make our lives better through competition
