What to monitor?

[StevenSeas]

I am in the market for a controller, keep debating between RKL, RKE, or Apex, I am thinking RKL right now. But What do you think is important to use a controller to monitor? I know i am going to do temp and 99% pH, but its up in the air between salinity and ORP. I have a JBJ ATO so ideally my salinity should stay stable but then there are water changes, and I havent ever trusted hydrometers very much.
So since as of now I can just afford one of these who thinks ORP and who thinks Salinity? Do I add something new to what Im testing and stick with my hydrometer? Or do I keep new and improved tabs on my ORP? Hopefully down the road i can get w/e probe I dont get now but I just am not in that kinda place finacially right now. I am kinda leaning towards ORP because its cheaper and when the values start to go down it is an indicator for a water change.
BUT i want to hear yall thoughts on the subject.