I'm trying to decide if these are worthwhile getting and if getting both is redundant. We have three days and want to do the hop on/off bus, Louvre plus one other museum. Probably will use a mix of the hop off bus and Metro to get from A to B.
Paris Visite passes are very poor value for money unless you really do use all the discounts, which you may not have time to do. Ordinary Metro day passes are quite poor value for money themselves compared to other cities. The best travel choice is to get a "carnet" of ten single tickets, which are cheaper than buying tickets separately, don't expire, and are usually better value than day passes.
I agree with Philip. For just a three-day visit buy ten-ticket carnets (€12.70) as needed and share the tickets. Each ticket is good anywhere the Metro goes (some lines end in zone 3) and on the RER in zone 1 (all of central Paris). A ticket provides unlimited transfers until you exit the system. A ticket provides bus transfers up to 90 minutes after boarding the first bus, but you cannot transfer between bus and Metro/RER on a single ticket. A ticket is also good on the Montmartre funiculaire that goes up the hill to Sacre-Coeur. Buy individual admission tickets for the Louvre and your second museum.