When we bought our house, it only had propane heat. After the first winter (2009), and getting $400-$500 propane bills each month (we didn't own our tank at that time), we made the plunge and bought a heat pump. For ours, roughly 33 degrees is the temp in which it switches over to propane. Below that temp, propane is just more efficient. We also bought our propane tank so we can shop for the lowest cost. Our house is 1100 square feet, and in the summer we keep it at 69-70 degrees and in the winter, we keep it around 70-71. We now only get two propane fills each year (250 gal tank), and were able to find a place that consistently has it for $1.79-$1.99/gal, and that is with being on their auto-fill program. Also, our PSE bill has never been over $125/month since making the change. Most PSE bills are under $100.
I should mention, our hot water tank and range are both propane, so those contribute to the use, and take away from the electric bill.