How we think about heaters
Tank size sets wattage (5–7 watts per gallon in a warm room, 7–10 in a cold one). Above that, reliability is the differentiator — one heater failure can empty a tank. Glass heaters with separate thermostats (Eheim Jager) are the long-standing reliable pick; flat heaters (Cobalt Neo-Therm) are better for nano tanks.
For tanks over 40 gallons, running two smaller heaters instead of one big one adds redundancy.