They are both heated with natural gas. The larger home has a single boiler system which heats the water, the radiant in floor heat in the lower level and the forced air in the upper level....it is also passive solar. The smaller home is a regular high efficiency forced air system.
The real secret is 2x6 construction with massive amounts of insulation. If our homes in California has been built this well, we never would have had to run the heat. I have always thought that better construction and more insulation is the low hanging fruit in energy conservation. It's such a waste to see all these huge energy bills in places that are MUCH warmer than Minnesnowta.
I think I win so far (or lose, really). Got our most recent bill while at Disney. $701. Our entire house is electric (we don't have natural gas in our area - only other option is converting to oil or propane). We keep the thermostat at 68 in winter and 72 in summer. The house is about 3500 square feet. During the summer, even when it's 95+ outside, the bill rarely gets higher than 175. So it's the heating that does it. We've added insulation to the attic, which has helped a bit. We also turn off the heated drying feature on the dishwasher, which shaved off about $50 per month, if you can believe it (we're a family of six, so it gets run a great deal).