In the process of just starting a 3100 sq ft house and we have been over and over this issue.  Ended up we are going with a heat pump with a propane furnace backup.  Heat pump will run down to 35-40 degrees outside temp and then the furnace will kick on.  Upfront cost increase over propane furnace and A/C alone is $500 as heat pump also functions as an A/C.  Annual savings should be around 25% over just propane.  Geothermal would have saved us 50% over geothermal, but would have cost $6,000 more upfront including the tax credit.  Everybody's situation is different.  There is a pretty good chance we will have access to gas in the next 5 years, so that is part of the reason we went the way we did.