Off-peak one-day travelcard prices:
2-zone > �4.90
4-zone > �5.40
6-zone > �6.30
Oyster Card:
Fee to obtain card = �3.
(Add on as much credit as you think you'll need).
Amounts deducted from your credit are as follows (at off-peak times, including wekends):
Tube in Zone 1 (or Zones 1 & 2) > �1.50
Tube Zones 1 to 'beyond 2' > �2
Tube not including Zone 1 > �1
Bus anywhere �0.80
BUT the maximum amount of credit which will be deducted in any (off-peak) day is as follows:
Zone 1 (or 1 & 2) only > �4.40
Zones 1 to 3 or 4 > �4.90
Zones 1 to 5 or 6 > �5.80
Assuming that you only travel around the main central London attractions, 2 Travelcards will total �9.80. An Oyster card would reqire an initial payment of �11.80 but �3 of that is refundable.
If you'll be making future visits to London, I'd recommend the Oyster card option. (There is no time limit when credit expires). If this will be your only visit, there's not much difference in the prices.
One last point: If your visit to LOndon might include a river trip, you should buy Travelcards. Many of the boat operators give a third off to Travelcard holders but people with pay-as-you-go Oystercards don't benefit from this offer.
Chris