Just my two cents.
I've used both the Apex and Profilux throughout the years and without a shadow of a doubt the Profilux came on top.
I have trusted in the Profilux for over 8 years now and not once did the system ever fail. I started with the P2 Plus, then later upgraded to a P3.1, and will eventually get the new Profilux 4 when it comes out. The system is rock solid and easy to setup. As Reefwiser mentioned, the Profilux involves using drop down boxes, no coding is needed to get up and running.
Aside from the reliability it provides, the build quality is top notch. Just holding the unit in your hands goes to show the level of detail and work that went into designing the product. It certainly doesn't look like a kids toy.
Another reason why I chose the Profilux is simply because I don't have to deal with little boxes and wires. Everything is nicely contained in one box and even includes additional slots to expand the controller even further. If there ever comes a time when I need to add another pH probe, I just buy the expansion card, pop it into the controller, connect the probe, and calibrate. No need for me to go out an buy an additional module, then find a spot to mount it. The design of the unit as a whole just saves me lots of space.
Do some searching around and see what kinds of problems people have had with the GHL controller, you'll have a hard time finding anything that doesn't stem from user error. Take a look at the problems Apex users have had with the new version and well...the results speak for themselves. Their classic version was good, but their latest release has people running into issues left and right. A good friend of mine who owns an aquarium service company just had the new Apex fail for the 4th time, I wish I were kidding, but it's true, everything ended up dying this time around.
Of course, some are user related errors, but there are also hardware issues too. IMO, it's kind of scary to think about when we put thousands of dollars into our tanks.
Support from both companies have a good reputation. Speaking from GHL's support, they've come a long way. They've made getting support easier than ever. I've heard of people getting support on the weekends and even holidays!