You're going to pay either way. Electric space heaters are very expensive to run. Keep the heat at 68. Also have the furnace serviced. It needs the fuel filter changed yearly and burner/burner chamber cleaned. A good oil furnace tech can get that thing running in the high 80s for efficiency. Look into HEEHRA. They are offering big incentives for heat pumps this year which are way more efficient than resistive heat space heaters.