It all depends on your system and where you live. Since you're in Neb I would go for the heater, it will keep your temps stable.
But run it for a few days, see what the average temp is at various times throughout the day. I don't have a heater in my tank, and my temps stay between 78-80. I have my AC set at keep the room at a constant 75 degrees and I live in S. Florida.