Oil cooler heater workes fine on a hot day when the oil is hot.... but wait a min.. then you dont any extra heat because it is a hot day already
, and on a cold day it does not work when you need it most.
The muffler heater is great and far better, its buring hot all the time and ready to give some heat right after start, at least alot quicker than the oil thingy
and it can almost keep the pilots blood above zero when flying with one door off.
if I would need to choose between them, I would go for the muffler type on any day.