Metro carnet or Passe Navigo?

Depends on how much travel you plan on, what day of the week you arrive, if you arrive and/or depart from CDG, and if so if you plan to use it on the RER or taking a taxi.

We will be using it within Paris. We arrive on a Monday. I just noticed that there are zones with the Navigo. We are only traveling within Paris.

Your only choice for the Navigo is the all-zone pass, the others are limited to the suburbs only. So if you are not using it for the airport, or Versailles, or Disneyland, it still could come in handy. At 22.80 EUR for a week. that is the equivalent of 15 trips at the carnet price of 1.49 EUR per ride. So if you plan on at least 2 trips a day, you are almost at break even, The Navigo is very convenient, with no paper tickets to hang on to and keep track of. It also makes transfers from Metro to bus seamless instead of burning 2 t+ tickets.

Get it on arrival and see how much you use it. If you don't find yourself using it 2 or 3 times a day then don't buy passes the second two weeks. If your lodging is very central and you organize your touring by area as you should then you are better off walking and then using the metro to get to distant areas e.g. Pere Lachaise, Eiffel Tower, Montmartre, Butte aux Cailles etc. With 3 weeks I would want to be doing day trips in the region and the ND is great for those in the Ile de France e.g. Vaux le Vicomte, Fontainebleau, Versailles, Malmaison, Provins, Crecy la Chapelle, Moret su Loing, the guinquettes on the Marne and Seine outside Paris. Here is one we had a wonderful time at one sunny Sunday afternoon: https://guinguette-auvergnate.fr

And note that what you want is not the Passe Navigo but the Navigo Decouverte. The former is only available to residents and is free, the latter is available to anyone and costs 5 Euro for the card. You then load a daily, weekly or monthly pass to the card.

I love how convenient the pass is. So much easier than handling paper tickets for me.

An electronic version of the carnet tickets was supposed to be introduced this spring but I don't know if the change is on schedule.
Meanwhile, single-fare tickets are sold by some Tabac shops, to avoid lineups.

When you’re using the carnet of tickets, you will pause and consider if you should use a ticket or just walk. When you have the Navigo, there’s no pause, you have the freedom (economically & mentally) to just go!

When I’ve purchased the Navigo, I really enjoy that I can go anywhere in Paris and then just hop a metro to relax at the Luxembourg garden with a gelato (from Amorino!) for 30-60 minutes before heading someplace else. When I’m using the carnet, i don’t move around as much.

Jean -- that is what I like best too -- it is such a cheap pass and it covers everything in the Ile de France -- I don't care if I save money in any given week since just not having to give it a thought is such a value. I have when staying in Paris a couple of months used it to hop on a bus for 6 blocks when it was pouring rain which I would never do if 'counting tickets'

If you opt for paper tickets, be sure to hold onto it until you’ve exited the mêtro. You can be stopped by mêtro police and asked to produce it. If you’ve already thrown it out you’ll be fined on the spot.

Right, Susan, and it's also important to discard (responsibly) the ticket as you leave the station so you don't mistakenly try to use it again and incur the wrath of the ticket booth guy when it won't move the turnstile. (I know whereof I speak.) ;-)

Dick, i’ve done it too. Embarrassing... : )

We (family of 4) just purchased the Navigo yesterday. It will probably be slightly more expenisive overall but the convenience is so great! I copied our passport photos at home (at 65% of size for a perfect fit on the passe) and brought them with me in an envelope. We bought them it the ticket window at the metro stop close to our apartment. It took a few minutes because they have to run each passe as a separate transaction, but now we can just jump on the metro anytime. We ended up riding three times yesterday - two of those I suspect we would have walked before to save the ticket. My teens are learning how to use the metro better, as well.

Be sure that your name is clearly printed in the space provided as a Navigo (or ND) card is non transferable.

A Navigo or a Navigo Découverte are simply blank cards onto which a fare of some type must be loaded. Navigo fare options are growing as the RATP gradually moves to paperless tickets. Not only are there various areas of coverage options, there are also fare choices for one day, one week, or one month.

The option of Navigo cards themselves is expanding as the Navigo Easy (supposedly available this month) and the Navigo Liberté enter the marketplace.