Hi,
The choice of Oyster or Travelcard really depends upon what areas of London you intend to visit and at what times. The max daily amount that gets deducted from an Oyster card is the higher value if any of your travel takes place before 09:30 on a weekday whereas the Travelcard is a fixed amount no matter what time you use it ... conversely if you only travel a little in the day you only pay that amount on the Oyster card whereas the Travelcard amount is again fixed.
As Newbie mentions its worth a look at the Tfl web site.
Note that you do not need to order the cards in advance .. you can pick them up in London.
Ask if you need more info.