If staying in either London or Paris for a number of days, is it worth buying a London Pass with Travel Card for savings on entry into various tourist spots?
Based on my visits to both London and Paris and the things I wanted to see, the answer is "no" to either the London Pass or the Paris Pass. The problem with both the London Pass and the Paris Pass is that the travel portion is pricey and both cover more zones than you are likely to need. You can do better with Oyster cards in London and ten-ticket carnets in Paris. Many major museums in London are free. The Paris museum pass can be a good deal if you're an art lover (like me).
This TripAdvisor thread will help you calculate potential savings (or losses) with the London Pass. http://www.tripadvisor.com/ShowTopic-g186338-i17-k4496157-London_Pass_Days_Out_Spreadsheet-London_England.html Be sure to check out the related Google spreadsheet and the list of free attractions.