Leaving the house in the fall—AC or heat?

We are leaving our house for about five weeks in New England during September/October. Weather could be anywhere from 80 during the day to probably around 40 at night when we return. What’s the most frugal setting for our central system? Cool at 78? Heat at 60? I am so torn about this and terrified of a huge bill upon our return should we make the wrong decision!! Help!!! TIA!