My preference is running 2 heaters in the lower end of the spectrum. You can always have a back up in the storage.
My fear is over heating water issues. I have had this happen in my macro algae tank and it killed my enormous Mexican Turbo Snails and a few other things. Once on a different time, heater was stuck on high but because I used 2 lower Watt heaters (in one tank) it raised the tank temp but not to a deadly level.
There are heaters now with a trip fuse sort of thing, so if it sticks on high it shuts down at a certain temp.
Just Aquarium Advice as far I know.