Ditto over on r2r has a huge thread on this, including functionality comparisons, personal experiences and there are inputs from Terence from apex and Matthias from GHL. It’s probably the most in-depth comparison out there...
Posters in this thread will likely support the brand they bought...and I’m one of them lol...
I use GHL, at the time I bought and did my research 3 things were deal clinchers for me, no apex eb8 in the UK, GHL had just launched there new controller the P4 so it was the latest spec and with all of the accessories ready for the yak market, but probably the main one was reliability, for me the apex appears to have far more serious “bricking” issues than GHL, now was this because the have a higher market share? Possibly, but I wasn’t going to take the risk. GHL have sorted out there cloud based interfaces and are now up there with APEX, you will have to learn either systems programming methods and each have plus and minuses, functionality is about even, dosing is more flexible with GHL with there precision dower 2.1 and larger volume Maxi doser. Both will control major brand Powerheads, (you will have to buy accessories to do this) they both control 1-10v devices. GHL have the KH director, apex the trident (when it arrives, and sort out the maintenance package you will need, in the states you can either return to manufacturer or choose to do the maintenance yourself, be interesting how European warranties view that) GHL will soon be launching the ION, which will measure and control Ca, Mg and measure NO3, they are trying to measure PO4, but are not ready yet and that will probably follow in 2020, this unit has the potential to be expanded to Fe, Cu and others. Both are similarly priced... I’ve tried to be objective but I did go GHL...