Benefit of two heaters

[The Guy]

Originally Posted by sphelps

It would take a while for a tank to cool off enough to cause problems, however for some reason heaters rarely fail in the off position. For this reason two small heaters is better than one large heater, however two large heaters is worse than one large heater. Hopefully that makes sense but ideally if a heater fails in the on position it shouldn't be able to cook the tank. Not so much an issue with external controllers.

I totally agree with this thinking and have never had a heater fail off when it dies, I only run a 125w and a 100w in my 90g with a 50g sump and it keeps my system at 80f. I plan on putting an external controller and make everything safer though. So I would say bigger isn't better in this case.