It's publicised that if you use contactless to pay for travel in London, it's the same price as using an Oyster card. But it's not always the case. We went out onto the tube, and made identical journeys using the two different methods of payment to prove what happens, and show you why the fares are different.
Thanks to Mike Whitaker and his website Oyster Rail, which is full of invaluable information on payment and fares.
Of course, if you have a railcard discount (or similar) applied to your Oyster, that will always be cheaper than contactless. Discounts cannot be applied to contactless payment cards.
For more details on capping, including how contactless has automatic weekly capping (and Oyster doesn't) that's here on the TfL website.