asmac, I don't recall any meaningful difference in the rate of tire pressure loss after repairing a puncture and using CO2 to inflate, at least when comparing the un-repaired tire's pressure to the repaired tire's pressure when the tires were next topped off.

Nitrogen is the largest molecule in the air we breathe and it also has lower water content than the air we breathe. This is good for tires that live through wide temperature changes, like an airliner or a racing motorcycle. The larger molecule also reduces pressure loss over a greater time span than the air we breathe.

Brad