[puddleglum]

IMO, the tank style heater is the best choice. It uses more power, but it also heats quicker and hotter than other types, so you don't need it on as long. I think the cost/degree temp. rise would be about the same, maybe less. I read on another thread that a member with a Prius can bring his car to full operating temp with a 1000watt tank heater in about an hour. The catch is you need to plumb it into a block drain for it to be most efficient. Using the heater hoses may work on some vehicles, but not well at all on others. That's why I don't have one, I have no block drain on my car.
Overall efficiency will depend on your setup and energy cost in your local area. I did some quick testing last spring (need to do more maybe) , but, it seemed to me that there was no visible overall cost savings in plugging my car in above around 40-50F. That was just my car, though, others may have better results. Also, I have no way to quantify the cost of wear caused by short tripping a cold engine.