Low thrust ships are interplanetary range spaceships that can be matter printed with little effort, and are popular with independent, isolationist, and low tech space faring groups. They have high ISP (range, fuel economy) but very poor acceleration compared to both short range chemical rockets and ultratech torchships.
These ships are mostly small (in the 100 to 1000 ton range) and powered by a basic nuclear fusion, or in low tech instances fission reactor.
Acceleration is generally in the 0.005 to 0.01 g range, occasionally upto 0.1g.
Ion engines use a beam of electromagnetically accelerated charged particles. They are safe, reliable, fuel efficient, low tech, cheap and easy to use, with few or no moving parts. On the down side, they produce only minuscule amounts of thrust. The more powerful ion engines have a fission or fusion reactor, and compare favourably with nuclear engines.
Nuclear engines use a cheap reaction mass (say, water, or hydrogen) heated by a fission or fusion reactor and expelled at high temperature out the nozzle. The rocket itself is similar to the chemical rocket, but the reactor and the exhaust plume is usually radioactive, this means they can’t be used to close to habitats. They have less thrust than chemical rockets but more than ion engines, as far as efficiency goes they are likewise intermediate. They are popular with isolated or libertarian groups who don’t want to rely on galactic infrastructure