Beergas is more expensive than co2, so I would carb with co2 and then push with the mix. The idea is the same amount of co2 should be used l, but with the mix you need to adjust for the fraction.
While one would want to carb lower, that is mostly due to the reason nitro was created. Somebody can correct me if I'm wrong, but nitro beers were created to mimic real ale while having a more shelf stable product. The higher pressure is needed to push through the plate which knocks almost all of the co2 out of solution. This creates a similar effect to a beer engine when pumped by a person that knows what he or she is doing. If straight co2 was used the beer would be crazy acidic and messy.