You will definitely need to rent a car. The public transportation in San Diego really isn't that well structured. The buses are fairly slow, and the trolley system only covers a small part of the city. You will definitely need a car to get around.
We have no subways here. You could take the trolley, but it only covers part of San Diego. I would suggest renting a car because San Diego is a big city and involves not only the city limits, but the outer areas of the county if you want to be able to see some of the really good sights such as the Wild Animal Park in north San Diego county (Escondido/San Pasqual area) or La Jolla Shores. Some of the places you may want to go to aren't all that accessible by bus. Plus, you need to know where you want to go and plan your bus routes ahead of time. I guess it will depend upon your budget whether you will choose a bus or rental car, but if you can afford to, I'd go with the car.
Unless you are going to stay in one area, you will need to rent a car. There is no subway system and the bus will take forever to go anywhere. This is especially true on weekends because I think the buses come every 45 minutes.
Yes, a rental car is a must in San Diego if you want see the beaches, Zoo, Sea World etc. without waiting for a bus or using the light rail (Trolley) that does Not travel to most of these sights. Bus system is "OK", but you'd have to take several transfers just to get over to Mission Beach or more to get to La Jolla or Coronado...
there is public transit there and you dont need to rent a car
they have day tripper passes which are valid on the trolley as
well as all SD transit buses
a pretty good deal