We'll be in Paris from a Thursday to Thursday. Which kind of metro pass should we buy?

Carnet - otherwise you end up buying two weekly passes for a one week period (Carte Orange passes are good from Mon to Sun).

Bardo is right unless you plan on taking the metro a lot. We did, and came out cheaper with the Carte, even though we didn't stay the whole seven days. BTW, on another post it mentioned that carnets are going up from 10 to 10.50 euro.

Remember you have to have a photo to go with the Carte. A school type picture will do; you don't have to spend extra on more passport photos.

You can't use the Carte Orange until Monday. Start with a carnet the first day - if you find you walk a lot and don't use metro much, then stick with Carnet a couple more days. Then you can decide if you need to buy a C.O. for Mon-Thurs use (it will not cover a trip to the airport).

If you decide you would like a day-pass after using your carnet tickets, you can buy a Mobilis for around 5-6 E. (meaning you'd need to be using Metro at least 6 times/day to warrant a pass).

I agree with just buying carnets or a Mobilis on real heavy-use days after you've figured out your style for a few days.

A Carte Orange will take you to the airport if you buy one for the right zones, but I certainly wouldn't if that's the only place you plan to go outside central Paris as it will cost a lot more. A Mobilis is now 5,30 euro for zones 1-2. I do like the Mobilis because it's unlimited and I like to use the bus a lot. For the bus, you need a single ticket for each ride, even if you plan to transfer buses. I really like taking the bus more in the summer, also, as the metro can be almost unbearable.

Also consider that the carnet tickets can be used by any number of people, while each individual will need a Carte Orange.