If this is your first trip to Paris, it is very difficult to know how much you will be using the public transportation system (metro tickets are also good for the buses). If you are staying in the center of Paris, probably you should just stick to buying carnets of 10 tickets as required (12€ now and 12.50€ starting July 1st).
Depending how many metro trips you will take, there arecseveral options explained on the Paris metro site http://www.ratp.fr/en/ratp/c_21890/discovering-paris/ . For a similar trip, we purchased a 5 day zone 1-3 pass for 14.95 E each plus a carnet (a pack of 10 tickets) for the remaining 2 days. The website gives prices prices for comparison.
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