I got a PS-30M, not because I expect more than 15A of panels, but because it allowed me to wire the trailer loads through the controller without exceeding its capacity.
That lets you monitor your current load using the controller's meter. With everything on (fans, lights, fridge, furnace, etc.), my total load could potentially go over 15 A. Some people wire them in with just the panel and battery connections, which is fine, too - you just lose the ability to monitor the load.
The PS meter show battery voltage, solar current, and load current. If you know both currents, it's just a matter of subtraction to know how much current is going into the battery, which I find useful. The battery current is not included in the displayed load current.
Plus, the 30 A one is only about $20 more than the 15 A one when I was shopping. I'm not aware of any advantage for the 15 A one, other than price.
edit: definitely plan on using the remote voltage/temperature capabilities of the controller. Your battery will appreciate it.