It depends on your circumstances and preferences.
Pay as you go with contactless and pay as you go with Oyster usually cost the same. However, pay as you go with Oyster is not valid for travel to and from all the new pay as you go with contactless stations.
If you have a Railcard or other discount, it may be cheaper to use your Oyster or Smartcard as you cannot apply a discount to pay as you go with contactless. However, you can travel further beyond London using pay as you go with contactless than you can with an Oyster card. Plus, there is no need to top up with a contactless card or device.