There are two main ways the fire can get the heat into your room, one is a convector the other radiant. Convector draws cool air in underneath and puts hot air out through the top of the fire, using the basic principal that heat rises.

A radiant fire relies on the ceramics in the fire bed and the hot box lined with ceramic fibre to radiate the heat into the room.

The only thing that matters in the choice of your fire is the output of the fire rather that the way it gets the heat into your room.