I too am considering a concversion to a GS heatpump from the air-air units. The direct bury freon-copper scheme from what I have found seems to be more of a problem. Locally I found several installers that tried, and will no longer use this system. I do not know the brand(s) in question, but there is a problem of copper corrosion in certain soils. Even a coated copper tube can be damaged in installation, or by rocks, and potentially corrode through. However the bigger problem seems to be a basic design issue. The freon line, during heating, has temperatures that are quite low. The soil actually has a relatively poor heat flow (which is why you need such long trenches in conventional systems), and the poor heat flow results in localized freezing of the soil around the freon line. This first of all will lower the capacity of the unit (due to a reduced differential temperature between freon and earth) and causes what was called "tundra effect". I heard of a number of installations, that were correct "by the book", that resulted in large frozen areas of the ground. Common complaints were that any precipitation would freeze on that part of the yard, from fall to spring. One guy had it installed under what became his driveway, which was iced for 3 or 4 months a year. It seems that it is not practical to really spread out the length of tubing that the freon line should slowly absorb heat, to the several hundred feet needed for ground heat flow. The result is a real cold spot.
I would try to get referenced (best not from the mfg direct) on installations and ask how it has worked. The antifreeze solution with plastic tubing, while more complex, seems to better match the thermal characteristics of the ground, and is a better overall solution.
I am considering a system from ClimateMaster.
paul