The geothermal system will be by far the most expensive and may not be worth it but you would need to call a pro in your area to find out how feasible it is. With -15 temperatures geothermal can save a lot of money real fast. How much do you currently spend per year on oil, how much are you paying per gallon of oil and how much are your electric rates. With that information we can usually tell you a guesstimated savings using another heating system.
The mini splits are great such as the Mitsubishi FE-12/FE-18 with hyper heat module. You could always add one or two of these to supplement the oil. The FE12 system provides 12K BTU's of capacity down to an outside temperature of 5 degrees, anytime you get below the temperature that the mini splits can heat you use the oil as a backup.