With a contactless debit card, you should only pay the same the as the lowest Oyster fare.
Potentially lower, depending on how long you are staying and how much travelling you are doing. Oyster only does daily capping: once you have spent the price of a day travelcard further travel is free that day.
Contactless payment does weekly capping: once you have spent the price of a 7 day travelcard, further travel is free that week - and better, as I recall, that's any consecutive 7 days, whereas I think you can only buy 7 day travelcards for Monday to Sunday.
However, you can't load a 16-25 or 60+ railcard onto a contactless card and it's not likely you ever will be able to, so if you hold one of those, it's probably cheaper to use a real Oystercard and get the railcard loaded. There's some complex dance I can't recall the details of (as I'm no longer eligible for one) about the best way to buy underground tickets with an F&F railcard; perhaps losing any MSE cred I ever had, I never bothered, and just used standard Oysters even when travelling en famille.
I believe it's now easier to get a 12-16 Oyster for children who don't attend London schools than it used to be. I was very annoyed that it was easier to get a child Suica card when visiting Tokyo than a child Oyster when visiting London, and in the end gave up and just paid adult Oyster rates for my children. They now have Oysters with their 16-25 loaded.