If you are only thinking of using public transport occasionally (less than 4-5 times per day) and only in Paris, it is more interesting to opt for the ‘carnet’ of 10 t+ tickets.
If you are planning to use public transport on a regular basis in Paris only, it will be more interesting for you to purchase a Paris Visite Pass zones 1-3.
Finally, if you wish to use public transport on a regular basis in Paris and also in the inner suburbs (e.g. to go to Versailles, Disneyland or even the airports, we suggest that you purchase a Paris Visite Pass zones 1-5.
For details on travel zones, see the RATP map here