having one heater not multiple to reduce running cost
They are thermostatically controlled, so they will only use the amount of electricity needed to keep the tanks at the temperature you set them for. The total running cost will be the same.
If you do go for a sump you can use two heaters. (Two in each tank can be too ugly and hard to hide).
If you need (for instance) 200 Watts for the total water volume, you can use two 100W heaters. That way, if one fails, you still have some heating and it will take a while before the temp drops to a dangerous level, giving you time to replace the dead heater. And if one heater should stick 'on' it will take a long time for the tanks to overheat and again you can spot the problem and fix it before it becomes a disaster.
Heater failure is rare, but if you have a sump it's so easy to take this precaution against it.
For running cost, the size and number of heaters doesn't really matter as long as they are up to the job. It's a question of how much water you have (which will be a bit more with a sump) and how much warmer the water is than the room. Whatever plants and livestock you choose there will be a range of temperatures in which they will be happy. There's a lot to be said for running your tanks near the lower end of that range. Including;
lower heating costs
more oxygen can dissolve in the water, - good for livestock and biofiltration
more CO
2 can dissolve in the water, - good for plants
hth