I lived in the 1st arrondissement for a long time, and often went to Orly. I agree with other posters that getting to or from Orly via public transport is neither particularly quick, nor convenient, nor cheap. Especially given the facts that you have a large suitcase and that there are 2 of you, I would not hesitate and take a taxi. Just my 2 centimes.
JimJamJet.