It depends on what you are doing. A baloon can be used for a closed system in which there is no gas evolved in the reaction and a stream is used when there is gas evolved. In the latter case you can´t have a closed system. I haven´t seen N2O (nitrous oxide, laughing gas) been used as a inert gas because it can act as a oxidizing agent, but N2 (nitrogen) is the most commonly used inert gas because it reacts with very few things.