With all due respect Sean, you're in Florida and this guy is in Northern NY. Big climate difference. 'Average' costs can be misleading in my opinion. One big factor in how long it takes to recover cost is the electric rates which vary widely around the country. Another factor is the air temperature (heat sink) when the thing is running which affects efficiency of a heat pump. I agree with you on the heat exchanger statement, but you have to also consider the other components which are about the same as a central air conditioner. I admit I don't know the answer to this one but 'efficiency' to me would mean overall cost efficiency which includes non-recurring and recurring costs figured over a multi-year time frame. I used to use propane until the price went through the roof and now I use solar. Just my nickels worth.
Al