Sizing a heater
A 100-watt heater on a 20-gallon tank is conservative for most rooms. Cold rooms (basements, garages) may need two 75-watt heaters for redundancy — a single heater failure is the most common cause of fish loss.
The Eheim Jager is the old-reliable pick because its thermostat holds calibration over years. Cheap preset heaters fail, and when they fail they usually stick on — which cooks the tank. If you can only buy one thing, buy a reliable heater.
Two heaters vs one
The “two smaller heaters” strategy is worth the extra cost on any tank you’d cry over losing. Each heater is set to the target temp. If one fails cold, the other holds temperature. If one fails hot, the tank only gains half the heat.