I'm looking for advice on which Taco control valve (if applicable) to use for my upcoming system.  As you know, the thermostat is connected through the valve to the unit, and the end switch in the valve turns on the compressor once the valve has opened.  Climatemaster specifically references Taco 500 series valves in the manual, and they are a semi-local company and well known reliable valve. 

I'll be using 3/4" piping for each branch for my two units and 3/4" valves, so I'll refer to the model number from each series that is 3/4".  There is the Taco 556, the "geothermal" valve 556G, and the Taco 571. 

The 556 and 556G are labeled as high head with 150 ft pump head of water.  The 571 is listed as 65 ft pump head of water capable.  The only difference I can see in the literature between the 556 and 556G is that the 556g uses O-rings instead of bellows on the stem, and silicone bronze instead of brass/eng. bronze for the seat assembly - otherwise specs look the same.  Perhaps the difference seal materials are better for cold water as these valves are usually used for hot water applications. 

One confusing spec is the pump head.  I'm doing a standing column system with my existing well pump set at 300 feet in a 410' well.  I therefore technically have a lot more pump head than the 150' spec of the 556, although maybe I'm not comparing this right.  Looking at some competitors valves they don't even give this pump head spec.  I suppose the well pressure tank buffers/reduces the pump head applied.   

Any insight on the control valves would be appreciated.  Thanks. 

The valves' head capabilities refer to pressures anticipated across the valve, not the depth from which the water originates. Every valve has (or should have) a pressure rating. Info supplied is incomplete, but if you are tying geo to domestic water system 65' of head won't do. Convert feet of head to pressure in pounds by dividing feet by 2.31.

That makes more sense, but its also unfortunate as the 571 is half the price of the others, and I need 5 valves. Two valves after each unit for each stage so I'm not constantly using max required flow, and one more that will be automated on a temperature sensor for bleed control. 

I understand my initial misconception - I was looking at the pump head spec and thinking of my well pump head as thats my application, buts thats apples to oranges.  Now I realize the valve spec refers to the the pumping head of a typical use hydronic heating system, and the head of water in pipes located above the valve.  Taco states the 556/G for high head applications like high rise buildings.  In these cases you could have 10 floors of water filled pipes pushing back on the valve when its closed.  Or in geothermal you could have 500 feet of closed loop pipe at a higher elevation than the equipment and valve creating head. 

I won't have that - I'll only have a pressure controlled 30 psi running though the pipes all located in the basement, so when the system is off there is barely any pressure head pushing back. 

Both the 556 and 571 have a max working pressure of 125 psi, which obviously is never seen anywhere in my system after the pressure tank.  Yes, the geothermal system is being tied into the existing domestic piping following the pressure tank (with 30-50 switch), with discharge back to the same well through a new pipe.

So after this thought process, I don't see why the 571 wouldn't work.