Beezar,
I find in most cases if you keep at least a 3,000-4,000 gallon barrier between the mfg's rating and your pool you should be fine. I like to usually make it 5,000 gallons or so but again not a huge difference.
The Nano Digitial uses an RC-35 (SC-36) cell which is set to work up to 22,000 gallons.
The Pool Pilot Digital offers 3 cell sizes up to 52,000 gallons. The RC-35 (SC-36) cell with the Pool Pilot Digital can handle up to 35,000 gallons. This is because the software on the system allows you to adjust the power level going to the cell whereas the Digital Nano has a single power level (you can still adjust output from 1 - 100%, just not the power level).
The Pool Pilot Digital also has the capability of running as a digital pump timer.
If you have more questions please let us know.