I am traveling to London with my wife and 11 year old at the end of June. We will be there for 9 days. We plan to spend most of it in zones 1-2 but we do have one out of town travel day scheduled to Watford. I can't figure out which to buy, the Oyster card or the Travelcard. Any help would be appreciated. Thanks.
They are not mutually exclusive. The Oyster Card is ONE way to pay for your travel. You can load "pay-as-you-go" (PAYGO) cash on the card, or purchase travel cards on the Oyster card. For PAYGO- the Oyster will calculate the least expensive cost for your daily travel based on where/when you travel each day. Oysters are NOT eligible for the 2-for-1 admissions and other 'deal's that ARE available on PAPER travel cards purchased at a NATIONAL RAIL STAION. (see this website for details on the 2-4-1 http://www.daysoutguide.co.uk/2for1-london) It sounds like you might benefit from buying 7 day paper travel cards to take advantage of the 2-4-1 deals. You will need to bring (or buy when you get there) passport sized photos so they can make your free id card to go along with the travel card. Then you can either buy daily travel cards or used Oyster cards for the additional days or even just by point-to-point journey tickets if you won't be doing much travel on the other 2 days.
I don't know if Watford is part of the London tube system. If it is not, then you will want to purchase RAIL tickets.
Watford Junction (for the wizardy folk) is reached by a train from Euston and a pay as you go Oyster can be used as long as the traveler taps in at Euston, even if it is at a platform without a gate barrier, there is still the yellow tap-in pad on the way to the train, and tap out at Watford Junction - the bus is just outside. Watford Junction is the furthest zone out from London. It is in a special zone all by itself called Watford Junction, but if it were numbered it would be number 10.
One advantage of the Oyster card is that at the end of your trip you can turn it back in and get back any money remaining on the card as well as the deposit you pay when your purchase the card.
If you are interested in the Days Out 2 for 1, make sure you purchase your kid's ticket separately so you are getting adult admission free. I would get 7 day paper travelcards then use PAYG Oyster for the rest of the trip with 1 day cards for your child.
You could also apply in advance for a 11-15 Zip Oyster card which gives free bus travel and child rate PAYG on trains / underground. However, as it requires a £10 admin fee the balance might not be worth it - although it would last until your child was over 16, so could pay off on multiple trips.
Thank you for the help. We can't wait to get there.