I think the main thing against the Oyster card - apart from the fact that it isn't usable on a lot of suburban rails ervices yet - is the (modest) deposit; but it's worth having and keeping if you plan to go to London occasionally and it does save money
Cheers - thought about it last year and didn't get one - I reckon it will work out okay this time since we are staying slightly longer - and I may go again later this year.
There is one advantage to a paper travelcard over PAYG Oyster - it gives you 2 for 1 admission to various tourist attractions. That can easily outweigh the 50p a day saving with the Oyster.