I am taking a group to London/Paris next year. we only have about 4 days in London and 3 in Paris. How useful are the London Pass and the Paris Museum passes? If they are useful... who is the best company to buy from?
The Paris Museum Pass is worth it if you visit enough attractions within the allotted time period. For example, with a 2-day pass (30E), if you climb the Arc de Triomphe (8E), visit the Centre Pompidou (10E), see artistic masterpieces in the Louvre (8.5E), and appreciate impressionism in the Musée d'Orsay (7.5E) in two days, you save 4E in admission fees. The museum pass also allows you to skip normal ticket lines.
Information on purchasing Paris Museum Passes can be found here. You may buy them online, or you can save on shipping and buy them at the tourism offices/airports in Paris.
Most museums in London are free. This was a change several years ago. Occasionally, there are fees or registration required for special exhibits such as the First Emperor exhibit going on right now. Typically you can make a reservation for viewing online. Also - depending upon when you will be traveling London has a new outdoor art exhibit - The Grand Tour - quite fun actually walking about - http://www.thegrandtour.org.uk/index.html
As for the Paris museum pass I would recommend. Museum pass is available here in Paris - and you can purchase with your first visit to the Louvre for example.
The transport option with the London Pass is definitely NOT a good deal. You'll be better off buying a Transport for London Oyster card. See www.tfl.gov.uk. And in four days it's not likely that you will visit enough of the places that charge admission and are covered by the pass to offset the cost. You'll be better off just paying as you go.
As others have said, having a Paris Museum Pass can work to your advantage.