Decision is very simple and depends entirely n how you make the electricity in the boat.
If you have solar
generators, your charging
routine is primarily a slow and lowish amphour charge over a long period.
If you rely on charging
by running a generator
or an engine
, then your charging routine is a fast high amp charge over a short period.
The holding plate is designed for that fast high amp charge and thus creating a reservoir of ice to keep things cool.
The evaporator works better on a solar/wind as the main charging system, as it puts a small amount of power into the evaporator over the longer period.
If you are always opening the fridge, and losing a lot of the cool, then a holding plate is not such a good idea (i.e. dont use a holding plate with a front opening fridge!
However, if you frequently have stuff that needs to be frozen down quickly (or cooled quickly) e.g. another slab of beer!
then the evaporator may not have the power to meet your needs.