As with almost anything there are advantages and disadvantages for both methods.  Most of your decision will come down to what you are trying to accomplish with the system.

Continuous Run Pump
-Keeps the outdoor unit temperature consistent throughout the water jacket, does not allow for layering of the water)
-Keeps any heat exchangers in the system hot for immediate draw, such as domestic water (if you use a plate or sidearm you must have constant flow to heat the water or you can use a flow switch on the domestic side to also switch the pump on, however there will be a delay)
-Uses more electricity
-More wear & tear on the pump
-Simpler wiring
-Better freeze protection with water always moving you will pick up some heat from inside the home and even draw some heat from your existing indoor unit in the even the fire goes out in the outdoor unit.
-More heat loss because you are constantly circulated through the ground, etc where at least some heat loss takes place, so in fact you could burn more wood

Switched Run Pump
-Uses less electricity
-Less wear & tear on the pump
-Only have heat loss when a house/building is calling for heat
-Involves a switching relay/controller to turn the pump on/off
*-water can layer in the outdoor unit possibly allowing it to boil over
*-freeze protection is only an issue if you turn thermostats down at anytime, i.e. a buildings heat demand is satisfied and it is 70 degrees, you turn the thermostat down to 45 and leave for the night, now the water will lay stagnant until that building comes down to 50 degrees.  This will take at least several hours in an insulated structure, at that point depending on how deep your insulated pex piping is you could have a freeze issue


Looking at these separate issues unless you have an internal domestic coal in your outdoor unit that you use for heating domestic water, if you have a heat exchanger for domestic water on your main heat supply/return lines then the pump must be continuous.

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