NTP vs. NEP

By the dawn of the 22nd century intersystem travel was common place throughout the Sol system. With the revolutionary invention of the Ashfield fusion reactors mankind was able to put a colony on Mars and by the time that the colony was flourishing settlements big and small had popped up all over the solar system.
For much of the history of space travel humanity had made use of chemical-liquid-combustion rockets. While effective, they were crude and inefficient. Nuclear reactors developed around the same time presented a potential solution, but the engineering challenges of making a nuclear reactor compact and light enough for a starship, as well as the inherent volatility of such devices proved a major roadblock. Everything changed with the introduction of the Ashfield fusion reactor. The fusion reactor's efficiency, vastly superior to that of traditional fission reactors, allowed for a much smaller and more compact form factor which suddenly made nuclear propulsion systems on starships not only possible, but practical.
Modern starships flown today are usually going to be operating with one of two types of nuclear propulsion: either a Nuclear Thermal Propulsion system or a Nuclear Electric Propulsion system. Both are based on essentially the same idea, but offer different advantages and setbacks. NTP systems generate thrust by taking magnetically contained hydrogen plasma and passing it in pipes close to the reactor's interior. Heat energy from the reactor is transferred to the plasma, which is then run through a nozzle to create propulsion. Alternatively, with NEP systems, heat from the reactor is used to generate electricity (usually by creating steam and using the steam to power a generator). The electricity generated from the reactor is used to power all of the ships system, including powerful ion engines that use the electricity to ionize xenon pumped from the ship's fuel tanks which is then accelerated through the engine's thruster to generate thrust.
There are different advantages to each, but which system a ship uses generally boils down to the ship's role. NTP systems offer higher acceleration at the cost of a much lower Specific Thrust (a metric for measuring the amount of thrust an engine can produce proportionate to the amount of fuel needed to gain that thrust) than an NEP system. NEP systems also primarily use the energy made from the reactor to fuel thrust, and as a result can run a lot longer on the same mass of fuel as an NTP, at the cost of a slower acceleration rate. As a result, NTP systems (which also are usually more complicated and costly) are usually employed on shuttles and smaller inter-system cargo haulers. Ships who's missions and roles call for short running times and missions where acceleration is a factor. NEP systems are usually employed on larger interstellar vessels who spend long periods of time in space travel, and who's mission times aren't primarily concerned with rapid acceleration or maneuverability.