Travel card or oyster?

I'm in London, I'm going to be using the DLR to travel to Heron Quays, and then the tube from there to the West End. Then I will be going to a bar somewhere else which will require using tube and DLR again. And then I'll be using DLR once again to get home. I have an 18+ oyster card so I'm just wondering whether it would be better to buy a travel card on zones 1-2 for �4.90 or just simply use my oyster card. And does anyone know the rules of one day travel cards on oysters?