Where heating systems refer to the machinery that produces heat and hot water, heat emitters refer to the equipment through which the heat is delivered.
Oftentimes the type of heating system you choose will pair up with a particular emitter type. Heat pumps and underfloor heating are one such match, both working at low temperatures and operated by weather sensors and other heating controls, although heat pumps are also often paired with aluminium radiators which also work well at low temperatures.
Delivering high temperature water on demand are the traditional and condensing oil, gas or biomass boilers, or wood furnace, which are often paired with steel radiators.
Steel radiators are less expensive than aluminium and were the most widely available kind in the past. You will normally get an instant blast of heat with fossil fuel boilers but then when the heating is turned off, the radiators cool just as quickly. That said, fossil fuel boilers can work with underfloor heating too.
Ultimately the choice between radiators and underfloor heating is a personal one but know that with radiators, your energy assessor is likely to oversize them to ensure the heat pump or boiler is not overworked. Nowadays plumbers no longer take on the role of sizing radiators.
Air heating delivered through skirting outlets or ducts in the ceiling is another option, as are trench heaters which emit heat from the floor. Trench heaters are most commonly used in front of large expanses of glazing.
Electric heat emitters used to be an expensive option due to the cost to run but are increasingly becoming a more popular for those with very low heat requirements. They require low upfront cost and are straightforward to install. These are often used where there is no central heating, e.g. in the form of underfloor electric mats in bathrooms or spot radiators.