We have a 4 ton horizontal ground source heat pump. We also have a 2 ton air source heat pump. I can say that the ground source is more efficient the majority of the time, but cost quite a bit more to install. Ours is now 10 years old and the only maintenance we have ever done is replace a starter capacitor on the motor, our system has 26,033 hours on it. Our ground return is about 40F or 4C in winter to about 60F or 15C in summer. In summer I can circulate the field water through the coil in the furnace and cool the house that way as well consuming only 100 watts for the furnace fan and 70 watts for a circulation pump, which is how it warms the field up to 60F by the fall.
We got the Air Source for two main reasons, mostly becasue we can't really run the geo on on our solar PV system, but I can easily run the air source and secondly becasue we use "time of use" electrical rates we wanted 6 tons of cooling in summer. We use the air source mostly in the summer for added cooling, running the air source from PV during on peak or demand rates and some in the spring and fall when the outside temps are more mild. Once it gets colder than the incoming field I switch over to primarily the ground source.
I would agree that geothermal systems are a lot more expensive, more difficult to install, but not on the Inefficient due to resistance losses along the tubing. Our system has two 70 watt pumps (mostly for redundancy sake, usually only have one on) to circulate. I am quite sure our air source looses a LOT more in the fan moving all the air than the pump circulating water.
But I do agree that it would take a LOT longer to get ahead cost wise with a geothermal system compared to a modern air source. I might save 20% by running the geothermal compared to air source but it would take 15 years for that to pay off, which it likely will in our case.
Having said all that unless you like to mess with system to get the most out of them I would suggest an air source. For instance in summer we can cool the house and that excess heat is put in to our pre hot water tank instead of dumping it outside, I can even circulate it between our two hot water tanks and get 130 gallons of hot water heated up to 140F before switch back to the field to dump heat, but usually at that point I switch over to the air source. Again that involves me switch valves and tinkering with things to make it more efficient.
Since we are on the Leaf forums I can compare it from moving from a car that gets 15-20 mpg to one that gets 40 mpg (air source) or 50 mpg (ground source), but going from 40 mpg to 50 mpg triples the cost.