Sorry, saw the title and couldn't resist
It's probably worth talking to someone who sells them. They're paid to know that stuff. I had a really enlightening talk to one sales guy (had to wade through a couple first who had NO idea) & he led us to the right heater for us... and it wasn't the most expensive one either!
IRT your question about wattage, as I understand it the running cost is only affected by the wattage of the heater. On your power bill you've probably been told how much it costs you per kWh. If it costs for example 20c / kWh you are paying 20c for every 1000w you draw per hour. A 2000w heater with no thermostat will cost you 40c per hour to run, yeah? So regardless of the type of heater, the cost is the same, it's just that you want the heater that's going to be most effective at heating the room (in the quickest time - that's where you'll save money).
We have combinations of micathermic heaters, oil-filled heaters & fan heaters, depending on the space. The micathermic & oil heaters are apparently very similar in power draw & cost but I prefer the micathermic one - it just feels more efficient to me.