One would be if you run a ca reactor, which eventually most will have to unless running all softies. That or force mass evaporation. The AC3 and 3pro can run run the add on unit pcx1000? I think is the model. Then place the additional pH probe in duty with the ca reactor. If you don't monitor ORP, then the ac3 can be used with 2 pH probes.
Number of devices controlled is a biggie too. You can eat up more than 12 easily and still have to use timers.
And alternate power input enables a battery backup too.
The webserver built in the 3 and 3 pro enables you to control remotely as well as check it just for piece of mind. And it will send you a page or email if something is going wrong that you've programmed an alam for. i.e. in the summer if the temp gets up to 85, you'd get notified, then go log on from the web and maybe turn some devices off until you can get home and see whats up.