I'd have to agree with Oyster card, I was only there a week but the time it saved was great. Also it really wasn't that hard to get one, took both of us a total of 10 minutes + 5 minute wait in line. If 15 minutes is too long, well...
Plus, sometimes you get lucky and the entire Oyster system goes down and you get a free ride that day 😀 Happened once while I was there.