LOL
PWM - Pulse Width Modified = Bogart Trimetric & Zamp for most factory installed campers
MPPT - Maximum Power Point Tracking - used for higher voltage panels, or 12v panels in series. Arguably "better" technology, but a lot depends on implementation.
DRJ is using a EPSOLAR Tracer series MPPT. I am using a Victron series MPPT controller. Neither are as well designed as the Trimetric, but because we have higher power (lower cost per watt) panels, we had no choice.
The PWM units will read the incoming voltage and aim for about .5V higher than what the battery is currently reading at, and feed the max current of the panel into the battery.
MPPT units will also read the battery, but can stand between the battery and controller more effectively, and choose the best voltage * current output of the panel, then CONVERT that to what the battery needs and send it that.
A well designed PWM unit (aka Trimetric) will utilize the maximum current output of the panel, and is constrained by the voltage outputs of the panel and what the battery needs.
A MPPT unit will choose the best current and voltage from the panel, but may do a lousy job of converting the power to what the battery needs.
In many cases, the Trimetric will outperform a MPPT unit.
So why did I (and DRJ and others) go with MPPT? because the higher voltage panels are cheaper per watt, and you don't have to be overly concerned about voltage drop.
And why can a PWM unit serve two different solar arrays? It is looking only at the battery to see what the battery wants, and then chopping the OUTPUT of what it gets to feed the battery that.
The MPPT unit is trying to optimize what is RECEIVING from the panels and converting/translating that into what the battery needs. So, different panels, different translation requirements.
Make sense?