Definitely cheaper! Tube tickets without Oyster are a massive tourist tax. Single tube journey is £4 for cash, or £2.70 (peak) or £2.20 (off-peak) Oyster.
You can buy one online.
https://oyster.tfl.gov.uk/oyster/link/0005.do
You can add money, or put auto top-up on where you can tell it to add £5 say when you go below £5. It will then automatically top-up on tubes/buses etc. and never run out.
Also, if you do many journeys in a day, it automatically becomes the appropriate travelcard. Can also surrneder it and get your £3 back should you wish.