Copper, HDPE, or Pex?

Most geothermal ground loops use HDPE (Polyethylene) tubing with something like 1.5" or 2" diameter. It is extremely durable, with a 50 to 100-year service life.

Direct Exchange (DX) ground loops, which circulate freon from the heat pump (instead of water-based coolant from a heat exchanger) through the ground loop, use copper tubing. When burying copper in the ground, you have to be careful of acidic or "aggressive" soil which can corrode the copper. A pH below 5.5 is a concern, and would need to be dealt with by using a protective coating, a sacrificial anode, or perhaps treating the soil with lime (though I'm not sure that's an "approved" fix).

I've also seen pre-made DIY geothermal kits that use Pex (cross-linked Polyethylene) tubing with 1/2" or 3/4" diameter.

I haven't decided yet which one I should use. Since my wells will be short, I need to dump the heat out of the tubing into the soil quickly, in just a few feet. So it seems I should maximize thermal conductivity with copper tubing. Copper's thermal conductivity is many times that of Polyethylene, and a smaller diameter tubing will expose more warm coolant to the cold walls of the tubing in the short distance available in our wells (the ratio of tubing wall surface area to liquid volume is higher in a smaller-diameter tubing than it is an a larger-diameter pipe).

I could turn down the flow rate to give the water more time to acclimate to the underground temperature, but I've read that I should keep the flow turbulent to provide better heat transfer, so I can't go too low. Also, the slower the fluid, the longer it's exposed to ambient temperatures on its way back to the server cabinet.

I found a nice calculator to determine whether flow will be turbulent or laminar based on the size of pipe and flow rate here: http://www.gcisolutions.com/flow.html

The soil near our building is not very acidic (6.5 pH), so I don't think corrosion will be a problem. I could bend 1/2" or 5/8" copper tubing into a helical coil and get 20 or 30 feet of tubing per well for maximum heat transfer.

I'm just not sure yet if this is all necessary, or if the rate of heat transfer is limited more by the soil (which typically has half the thermal conductivity of polyethylene anyway). Maybe copper tubing would be a waste of time and money.