Problem with heating the oil pan is all the oil sits in the pan when the engine if off. That means heated oil is not circulating throughout the engine until you start it, and by then the damage is done. I like the simplicity of a block heater, and the effectiveness of circulating warm coolant throughout the engine.

As a side note, hotels and motels in northern Ontario (like Sudbury for example) have posts in the parking lot with electrical outlets for block heaters. Also, block heaters are standard equipment for any vehicle built for the Canadian market, versus being an option for vehicles built for the US market.