It depends on what zones you’ll be frequently traveling between. Look on a map of the places you plan to visit, and then see where they are positioned within the tube zones. Most people only use a couple zones, so your best bet is just to go to a ticketing window and tell them you want a pass good for a week good for the zones you’ll use. It will ultimately save you money rather than risk buying a pass for all zones, or fill an Oyster card with money you may not use up.