Generally, of the two I prefer Frommers. Fodors website is a great source of on-line information. However, for the Paris and London specifically I like Rick Steve's books. He has details of walking tours and self-guided tours through some of the museums, plus interesting little tid-bits of information that the other books often don't include. When I travel I often end up taking two or three books (Frommers, Lonely Planet, and one other if appropriate....) so picking only ONE book to take would really strain me.



As an aside - my in-laws just made their first overseas trip (to England) and didn't want to buy a guide book (they are very frugal). My husband and I gave them a Rick Steve's book for London and a Lonely Planet for England and they were surprised to find information that they couldn't get off the internet, details about how to travel from one location to another, historial information, etc. They have been happily converted to using guidebooks. In particular, the really liked Rick Steve's London book.