Every electrical appliance sold in Australia - including all electric heaters - must by law have a compliance plate. One of the pieces of information that must be on that plate (often a metallic sticker, or molded into the body of the appliance), is how much power the appliance uses.
This will be recorded in either watts (W) or amps (A). In both cases, the smaller the number, the less power the heater will use. It doesn't matter one little bit what kind of heater it is, the power rating tells you what it will cost to run while it is cycled on. (Some heaters have a built in temperature control that turns the heater off for part of the time.) While on, a 2000W ceramic heater costs exactly the same to run as a 2000W radiator, or a 2000W fan heater, or a 2000W laser beam! If you are comparing two heaters, and one is in watts while the other is in amps, simply multiply the amps reading by 240, and the result will be the watts rating of that heater. (Similarly, if you divide the watts rating by 240, that will be the amps rating.)
Now that covers running cost while they are on, but that isn't the same thing as efficiency. Just as important as the running cost, is that you have the right kind of heater for the place you want to use it in, and the time you will be wanting to use it.
"Convection" heaters are good at warming up air, but not very good at warming up objects. (Objects get 2nd hand heat from the air rather than the heater.) Since they have to heat the air before they can heat you, they are best used in places where heat will be required for long periods of time at a stretch. Also, they are only really a smart choice where you can contain the heat that they create in the air - so lack of insulation in the roof and walls, or significant drafts make a convection heater next to useless.
"Radiant" heaters are far better at warming up objects than they are at warming up the air. (The air gets 2nd hand heat from the objects rather than from the heater.) Whatever sits in front of a radiator will get warm almost immediately, irrespective of what insulation or draft may be present. That means radiators are the best choice for outdoor areas or rooms without insulation, or places where you will only be for a short time (so long as you turn the radiator off when you leave). A myth I constantly fight against is the idea that leaving a radiator on will heat up a room. It won't! The radiator might heat up an empty chair. With luck the empty chair might give enough of it's 2nd hand heat to the air to make a difference before it spontaneously bursts into flames and burns the room down. Even if it doesn't burst into flames, it is just about the least efficient possible way to heat a room. Just don't do it!
"Conductive" heaters are the most efficient of the lot, but the least practical in terms of room heating. Conductive heaters heat only those things that they touch. The most common example, would be the humble electric blanket. Just about the lowest power consumption (and hence running cost) possible, and it keeps you nice and warm without wasting any heat anywhere else. Just not great at Dinner Parties!
Finally, you need to take account of how much energy is required to actually heat the space you want to heat. This is where things get quite complicated, and for a precise calculation, you would be best off seeing a professional. Many air conditioning companies offer this kind of calculation as part of their sales technique, and often energy suppliers or even government regulators can point you in the right direction. Often, you can make physical changes to the area you are looking to heat that will reduce the amount of energy required to do the job. Things like changing the curtains, installing insulation, blocking draft entry points, and even what you plant outside the windows can all make a difference.
So, the bottom line on heater choice?... Start by choosing the right technology for where and how the heater will be used. Then figure out how much power is needed to do the job, and then go shopping for the right kind of heater that has the lowest power rating, that is still big enough for the job.