I did a lot of research and the cost difference isn’t insignificant. What I found on here and other forums is that the salinity probe seems to be the least reliable and gives people the most headaches. Also from what I’ve read the ORP data is really only useful if you’re considering running ozone (someone please correct me if there’s a different game-changing use for this).
In terms of salinity, assuming you get good reliable data, a case can definitely be made for that. An older former-Reefer friend lost his 300g probably $20K tank to a faulty ATO (had some kind of drain to waste emergency overflow so no flooding, SW just kept getting replaced with FW). Realtime salinity could have provided early warning and prevented this.
However, IMO this can be prevented using other means. Apex time-limited programming on either the ATK pump or just the ATO outlet watts (Tunze and others already have time-limited programming built in). High water sensor on the sump (does require a $99 module to support that but also can protect against other failures like clogged overflows, etc.). In my friend’s case, never rely on a simple float valve alone to begin with. Limit the total RO water available, etc., etc. I’ve found salinity to be incredibly stable under normal conditions and checking it every few weeks to once a month always reveals nothing to do.
So for my setup I bought 2 EL’s (DT and coral QT) and I’m comfortable with my failsafes. Better than having a questionable probe I constantly need to calibrate, and another probe I don’t even have a use for. All for shelling out a few extra hundred dollars that could go elsewhere.
To directly answer your question: the ONLY significant difference between the two is those two extra probes. Everything else is identical. And, you can even add those probes later if you want by buying the appropriate module and probes (but this probably costs a little extra over the full Apex purchase price). Trident, optical sensor module, every other module, no problems. I’m not saying don’t buy the full Apex; it’s the Cadillac model. But running the cost benefit analysis for myself led me to the EL’s.