Logic says as a base defense option you ought to go with the sigma field, slowing the zerg down, a building what must be left to the fore of your defences to stand a chance of drawing attention to and taking control of a zerg unit of power isn't exactly logical. If the Zerg is in your defensive perimeter the time taken to take over the unit means it will suffer most likely fatal damage. If you can manage to take it over however then awesome you could just trigger medics to heal them manually once the carnage dies down