A radiant radiator will allow heat to travel directly around the room.This allows the whole room to be heated all at once, and once it is at temperature, it stays that way. Convector radiatros work on a different principle. They use (funnily enough) convection to heat up the air. This happens by heating the lower layer of air. When the molecules get hotter, they move faster (increas in energy), causing the air to become less dense. This means that the hot air rises, and the cold air takes its place. This process is repeated, and the cold air continually replaces the warmer air around the radiator. This is slower and less efficient, ad it means that the air is constantly being re-heated. As for Inertia heaters... I think they heat the room by just blowing out as much heat as possible. That is a complete guess, due to the fact I have no idea what one is.
My suggestion would be to go for the Radiant Radiator.