Never ever buy the visitors versions of travel tickets or cards ANYWHERE, they are always way more expensive than the regular version.
I have a regular Oyster pay as you go, have had it for 3 years now. It cost £20 ie £3 oyster charge and £17 credit. I top up as I go and if at any time I want my £3 back I can hand the card back. The Oyster caps the fare at 50p less than a travelcard for all the zones visited in that day so you save. It's really handy and works on the buses too. I like to leave about £10 on it because sure then I have to go back, can't be wasting £10!