...efficiency and running costs...
On that, I don't think you'll see much difference. Any electric heating element is turning nearly 100% of its electrical power into heat.
We've got an oil-filled heater that we sometimes use in the bedroom when there's a north wind. I like that no part of the unit gets extremely hot -- it would take unusual skill to accidentally start a fire with it.
Although it's a "radiator", I suspect most of the heat doesn't get radiated -- instead, it warms the air. Which means a nice warm ceiling and a cold floor. If I were relying on one of these for heat, I'd probably add a quiet fan running on low, to keep the air mixed.