I have friends staying with me (in London) from Canada next week - would it be cheaper for them to get an Oyster card or a Daily Travel card when they go sight-seeing?
Depends on how many days they are using it. Generally speaking though an Oyster works out at better value, you will never pay more a day than you would for a travel card anyway, and you aren't tied (as you once were, dunno if it still applies) to starting journeys AFTER the morning peak. They're straighforward to get but involve a little paperwork before you use - just pop into an oyster seller to get info
also, I think you have to pay a deposit on them now and claim it back afterwards. So you may want to factor in whether you think the minor hassle is worth it. (Or you could always keep the card for yourself when they leave.) As Postdog says, the Oyster will never cost you more for a day's travel than a travelcard.
I forgot to add one of the advantages to an oyster is that assuming you don't use it up to the travel card cost, you still save on what you would pay for individual fares.
also unused money stays on the Oyster card for future use. A travelcard is worthless at the end of the day (or week or month or whatever) if you haven't used it.
Oyster Card fares used to have a daily cap which was 50p less than what the traveller would have paid for a Travelcard covering the same journeys. (So Oystercards used to be marginally cheaper). However the daily cap is now set at the same amount that it would have cost to use a Travelcard for the same journeys, so there's no difference (apart from the deposit required to obtain the Oyster Card) in the what you pay.
Wow! Thanks, Postdog, jno and Buenchico. I could not make head nor tail of the info on the tfL web-sight. I think the Oyster card will suit my friends down to the ground.