Go for the oil cooler heater, no chance of carbon monoxide poisoning,less maintenance cost.
The the oil cooler unit could be much hotter with insulated cabin feed pipe, the pipe must loose 30% of the heat.
Don't know if there is an engine oil heater for colder climates this would take care of the initial cold start problem & could prolong eng life
Think the heat exchanger exaust is multi thousands of $ range with on going maintenance requirement
