Hi James, there are two ways to sense the chlorine, traditionally REDOX the potential voltage in the water telling you chlorine is present measured in millivolts. This is pretty inaccurate as cyanuric acid amongst other things throws the reading out and when producing chlorine the system also produces gasses and these tell the sensor that the reading is going lower although it's actually going higher because you are making chlorine.
Amperometric sensors cost a lot more and are not affected by the gas production or cyanuric acid etc and measure an electric current across the water so giving a much more accurate reading. Why does that matter it saves run time on the unit so money on the replacement salt cell chamber.
As they say on this forum and should on many more Testing with a good test kit is still important even with these gizmo's
As for recommendations, not sure I am the best person to ask as the forum members on here have more experience than me with salt water generators (SWG) From my standpoint I look for switch mode power supplies as apposed to ordinary as they save around 40% of the electricty of a standard unit. I did like the look of the Pool Pilot as it's well thought out but the sole UK importer also sell their own branded units so as they have an exclusive deal with Pool Pilot they push their own units as they get a bigger markup and control the supply of Pool Pilot's in the UK, clever yes, but not an honest way to do business, I did email Pool Pilot but no response on that issue.