The choice of airports in Paris is slightly dependant on where you are planning on staying.
CDG is located to the north of the city, and ORY to the south. ORY is closer to central Paris and serves mostly domestic and European flights plus lots of charters. CDG is more of a intercontinental airport and is a hub for Air France.
Transportation from both airports is quite easy. Air France runs a bus service from both airports to various points in the city. From both ORY and CDG you can also take the RER train to various parts of the city.
So unless you are staying close to either airport, I guess your decision should be more based on the fare and airline you rather fly.
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