I have an Oyster - much easier than a travel card as unlike travel cards, it's reusable. It works on both buses and the underground and all you do is tap and go. Travel cards can wear out and are a pain in the arse when they do, and if it's lost you are out of luck where as with the Oyster - just register it online and your balance is safe if it gets lost.

I keep mine with my passport and have got much more use out of it than I would have a travel card.