There are 5 main differences between underfloor heating and radiators, these are heat distribution, comfort and air quality, cost, aesthetics and zoned heating.
Central heating with radiators was introduced in the 30's and has been used ever since for most UK homes and this hasn't changed for the last 8 decades. However, recently underfloor heating is being used as the main source of heating as it offers comfort and luxury.
The way heat is distributed in your home depends on what heating system you have. Underfloor heating produces radiant heat which makes you feel warm, this terms is used as radiation is responsible for the proportion of thermal comfort that you receive from underfloor heating.
Radiators heat the air surrounding them using convection. The air that is heated by a radiator rises towards the ceiling of your room and once it is cooled down it falls to the ground level to be heated by the convection of the radiator again. This creates a flow of hot and cold air but your room will have hot and cold spots in some areas.
Efficiency & Comfort of a System
The way that heat is distributed around your home will impact the efficiency and comfort of the heating system. Radiant heat will heat up objects directly and maintain the natural humidity in the room, where as convection of warm air tends to reduce the humidity which will make heated rooms a bit stuffy. if the whole floor of a room is heated then radiant heat gives a different spread of heat but conventional heating will heat one area first and then slowly reach the required comfort levels.
Overheating is Expensive and Inefficient
Radiators were a luxury back in the 30s, but they could overheat which makes a heating system less efficient, to make it better you would need ventilation like opening a window.
Radiators produce heat which causes hot and cold spots, to make sure you can feel the warm further away from the radiator you will need to turn up the room temperature. This will heat up the radiators and reduce the cold spots in the room but making the radiator hotter so you would have to open the windows to let fresh air in but by doing this you will have to let out the hot air. Wasting the energy is expensive, overheating by just 1% can increase your fuel costs by 8% which increases your heating bills.
Underfloor heating takes up no extra space in your home as all of the heat source is within the floor. Radiator systems are quite bulky and have to be inside the room to a wall. If your room is quite small then it could be quite problematic.
Zoning - Temperature Control in Each Room
Heating is more efficient when it is used zone by zone. Most the time, underfloor heating is used and zoned room by room with a thermostat that controls a specific zone so the heating is only supplied to the room when it is required, reducing the amount of energy. Radiators normally heat up throughout the whole house at once based on a central thermostat. This causes overheating and under-heating depending on the thermostat location.