Oyster Card

If I recall correctly, I put 15 pounds on each of ours for the 24 hours we were in London last summer and used every last bit of it. The Oyster card was great actually, having a unified public transportation system was great. Subway, buses, boat on the Thames...it was so convenient.

We also used the pay-as-you-go Oyster card and found it very cost effective and efficient. For five days, I put 35 pounds on it and had 7 leftover. It just depends on what you are doing. If you have money leftover, you can go to any station upon your departure from London and get the balance back, so don't fret too much about losing the leftover.

Keep in mind that if the balance on your Oyster card is low, you can add funds anytime during the week. There is no need to figure out the precise amount when you purchase. For example, you could start with 20 and add more later. However, for six full days, I would put a zone 1-2 weekly card on the Oyster plus a little extra (about 10 GBP) pay as you go money to cover your trips outside of zone 2. This assumes that you are staying within zone 1-2.

For 6 days you will be better off getting a 7 day Travelcard than a Pay As You Go Oyster. A 7 day Travelcard for the central zones 1&2 is 27.60 GBP.
The daily cap on Oyster is 6.60 GBP to 8:00 GBP depending on whether you travel peak or non-peak each day.

Thanks all. You have been very helpful..

Don't sweat all the permutations about what might or might not save you a few dollars. You won't know how many Tube/bus/rail trips you will take. Put 20 pounds on an Oyster Card. You won't regret.