Input power can be plentiful with modern solar cells, and Sun just keeps on providing 1 kW / m² in optical emission.
Propellant, OTOH, is finite; once you use it up, you cannot maneuver any more. Solar cells may degrade, but keep providing power for decades, if not centuries.
So a long-lasting satellite which just needs to keep its orbit may be better off with a low-thrust engine not ejecting any reaction mass. A long-range mission with a finite travel time towards the outer reaches of the Solar system may be better off with a high-Isp jet engine.
Propellant, OTOH, is finite; once you use it up, you cannot maneuver any more. Solar cells may degrade, but keep providing power for decades, if not centuries.
So a long-lasting satellite which just needs to keep its orbit may be better off with a low-thrust engine not ejecting any reaction mass. A long-range mission with a finite travel time towards the outer reaches of the Solar system may be better off with a high-Isp jet engine.