Personally, I would say a metro pass would be of more use. Its much easier to navigate your way round on the extensive metro system. Can't remember what a 'several day' pass is, but to give you an idea you can get a 10 trip ticket for �7.40. Tickets are available at the machines at all metro stations. If you want to 'sightsee' the Barcelona Turistic Hop On Hop Off bus runs 3 routes (but I think it goes down to 2 in Sept) and this costs �20/day or �26 for 2 days.
Go to the main Tourist Information office )under the square at Plaza de Catalunya) and get a "Barcelona City Card" which gets you free travel on the Metro, Buses and local trains as well as discounts (and some free offers) at just about all the major sights (Parc Guell, Sagrada Familia etc.) 4 days is �34;