Although this doesn't specifically answer your question, will you ride the tube enough to justify getting a card to save money?
My wife and I were in London for 6 days, we stayed in the Pimlico area, and we only rode the tube from the train station when we came in from Bath with our luggage the first day, once when it was getting late and we were several miles from our hotel, and to get to the train station for the next leg of our trip to Paris.
For us, one of the best parts of London was walking just about everywhere we went, and knowing that if we get tired, hungry, etc... there'd be a pub around the corner for a little break. Unless you plan on visiting sites that are quite a ways off the map, you can't beat walking. Just be sure to remember which direction your traffic is coming from when you go to cross a street!