Keeping warm is obviously a big issue for most of us. Many posts mention fan heaters as an option but I would go for a convector heater every time. We use a single convector heater and it keeps our Zenobia 6 warm. Much more efficient and effective than a fan heater I find.
I think it depends on how you use the heater. We have a fan heater which we use for a quick burst of heat morning and evening (no more than an hour at 1kw) if necessary.
Some people leave them on overnight, when perhaps a convector heater would be of more use. Mind you, if everyone starts doing that, expect EHU prices to increase substantially in the future.
I like fan heaters because they take up minimum space in the car.
I also think it's better to have the heat blown towards you in an environment like a tent where it tends to rise up and straight through the tent and into the air.
No heater is really much more 'efficient' than another. A 2KW fan heater gives out the same amount of heat as a 2KW convector, it's just delivered differently.