I hear alot about having 2 weaker ones is better. If one breaks it wont be powerfull enough to cook your aquarium.
If the filtration is good the water should get distributed evenly right? I think two heaters would work for you. I have read that power heads can also generate heat too.bentcountershaft said:On a tank that long I'd go with at least three or even four heaters spread around the tank to help keep the temperature evenly distributed.