The London Pass is a nice prepaid card that is scanned at the attractions for entry. It's easy to use, and covers most the big attractions, though it was missing a few. Most places had a special London pass entrance so you go to skip the big lines. We happened to visit the London Tower during the Queen's Boat Parade so having the pass let us into the attraction when the thousands of people around us couldn't, giving us a nice view of the parade. There are also some discounts at stores associated with the pass but I don't think we used any.
We did not get the transit pass because there are restrictions when you can use it (not during peak hours, only in certain zones). Also, using it when you first get there (to get to your hotel) activates the card and uses one of your days. We also chose to pick up the pass in London (pagoda like building in Leicester Square) so we would have had to still buy tube tickets to get the pass). Instead, get an Oyster Card for the London Underground. You can get to any big attraction on it and it is much easier to use than buses. Order a visitor's Oyster card online before you go and you're set. You can easily add money at the machines in the station if you run out. To use, just tap it on the green Oyster card pad at the turnstile and go!
If you're looking for simplicity I would definitely recommend the pass.
Connect with Facebook to get one click access to Trustpilot. It's easier.Sign up with Facebook