Both types can regulate charging from solar panel to battery, but they work differently. Therefore, I do not choose a controller based on price alone.
PWM – the simple solution
In principle, a PWM controller brings the solar panel closer to the battery's voltage level during charging. Therefore, it is most suitable for small systems where the panel and battery are selected to work closely together.
MPPT – works with the panel's optimal operating point
An MPPT controller continuously searches for the solar panel's maximum power point and then converts the energy to the voltage and current required by the battery. This makes it possible to have a higher panel voltage than the battery voltage, as long as the controller's limits are respected.
Why does this matter on a boat?
On a boat, space is often limited, cable runs can be long, and shade and temperature change. Therefore, it makes sense to look at the entire system: the panels' Voc and current, cable, battery, and the controller's maximum input voltage/charging current.
My short rule of thumb
For most modern boat systems, especially when there are multiple panels, higher panel voltage, or lithium batteries, I would start the assessment with MPPT. PWM can still be relevant in a small, simple system with appropriate panel voltage.
See solar charge controllers · How to choose the size of an MPPT