What's more efficient: Ammonia w/ Steel or R404a w/Copper

Hi all, back with another question. So I keep finding new variables that affect the efficiency of the refrigeration cycle differently. This time it is the thermal conductivity of metals used in construction of heat exchanging elements. Specifically I can't figure out which combination would result in a lower cost of production: ammonia with stainless-steel evaporator and condenser or R404a/R22 with copper (or some copper alloy) evaporator/condenser. Ammonia is a superior, natural refrigerant. But copper's thermal conductivity puts steel to shame. On balance, which combination would you pick? :confused:

All comes down to how much money you want to spend and achieve a suitable pay back recovery.
***** based refrigerants are history, so plan a plant based on natural refrigerants, to make it saleable back into the market place down the track when you want to step away.

In terms of heat transmission you have to consider not only the conductivity of the metal (tubes) but mostly the interior and exterior convection. This values influence much more the global heat transmission coefficient then the metal conductivity. Of course that finally there exists some influence.
It's only a matter of making the calculations...
I would bet on the ammonia + stainless steel ;)