I would offer two suggestions for your consideration. Well it is really one suggestions applied in two ways. I dislike having single filters or heaters on most tanks unless they are smaller. So I would suggest you might want to consider using two heaters of say 300w each. This has multiple benefits if the heaters are in the tank or the sump.
In the tank they help with circulation and by having two spread out, you get more even heat. But an even bigger benefit is if one fails, the tank will not be completely without a heater. It may cool down some but it wont likely get so cold as to harm/kill the fish.
Next, I normally have multiple filters on all tanks save those running on air power. I have central pumps for that. But, when possible, I try to have a hang on filter as well. This helps oxygenated the water and provides some backup if the pump fails. I also like to do the reverse and have an air powered foam in the tank even when a canister or power filter is the main unit for the tank. During a power outage of blink, a power filter may not restart whereas an air pump almost always will.
With a sump system things are different. Almost everything filtration related and even heaters may be in the sump. This also needs to have adequately oxygenated water for the bacteria. Most times folks use sumps as a way to keep equipment out of the main display tank. While there are many benefits to a sump, redundancy is not one of them. So you may want to consider having some aeration in the tank which would help for a while if the sump quit for any reason.
What is a Single Point of Failure?
A single point of failure (SPOF) is essentially a flaw in the design, configuration, or implementation of a system, circuit, or component that poses a potential risk because it could lead to a situation in which just one malfunction or fault causes the whole system to stop working.
It is impossible to protect against everything, but there is a certain amount of redundancy that makes sense to have. We all have to decide where we want to have two or more instead of one. For me it means a lot of multiple heaters and filters and a back-up generator. However, if that generator happens to fail when there is a power outage, I am toast.