Unless you insulate the sump and return plumbing (which I am not suggesting), there's a crazy amount of heat loss between the sump and display.
Because of that, and general safety*, I heartily recommend a heater in the display.
300 watts is kinda long, but if you can hide it well enough, then go for it and move than up to the display. Otherwise, you might prefer two 150 watt heaters that are only 12".
Them put a little 75 or 100 watt heater down in the sump.
If you keep the 300 watt heater in the sump, you can probably get away with a much smaller heater in the display, except for that emergency scenario where it might have to heat the whole tank.
You can also take measures to reduce evaporation – that'll cut back on heat loss from radiation as well as evaporative cooling.
2¢
* What if your return pump fails? It happens.