Museum Pass or Paris Pass?

The Paris Pass is not a 'pass' if by that you mean a single card that takes you to museums and on the metro -- it is a marketing scheme whereby the overpriced Paris Visite travel card and the ordinary Paris Museum Pass are put in an envelope with a few coupons and a hefty mark up.

The Paris Visite is almost always a terrible idea. The Museum Pass can be very useful on a first trip to allow you access to shorter security lines at places like the Orsay, Louvre and Orangerie and it also allows repeat visits which is nice in particular for the Louvre where two 3 hour visits are a lot more pleasant than one 6 hour visit. It also allows you to nip into places you might not otherwise pay to see for a brief look.

For transport. If you are there a week that falls between Monday and Sunday then a Navigo Decouverte pass is worth getting. Otherwise just buy a carnet of 10 ordinary metro/bus tickets for 14.10 and when you run out buy another. You can share them out with your companions. We use them to get to distant spots e.g. Eiffel tower, or Montmartre or Pere LaChaise, but then walk in the area we are visiting that day and often walk back to the center or use a second ticket to ride back. Paris is really best enjoyed on foot and so planning to use metro for long hauls and walk otherwise is both economical and more pleasant and you see the beauty of the city.

Great question and info!

Does the museum pass let you "skip the line'?

The Museum Pass lets you skip the line to buy day tickets. It does not let you skip the security line.

And the security line (which the pass doesn't let you skip) can be very long at some of the more popular attractions, like Versailles.

Not quite. Having tickets or pass allows you often to use short security lines. e.g. at the Orsay the regular line can be 2 hours long -- half an hour to an hour is standard. The security entrance for pass/ticket holders is often totally without a line, you walk just in; the worst I have seen it, we had to wait about 15 minutes in line. The regular line at that time was two hours.

At the Orangerie, same thing. There are two security lines, one for those with and without tickets. At the Louvre, the Caroussel used to be an easy entrance but publicizing it in forums like this now means it is also a zoo with long security lines. The pass lets you use the Richelieu entrance and avoid the long lines at the Caroussel and the Pyramid. Most places that take the pass give them some priority.

For once the cheaper option is the way to go. Thank you for the great advice.