The San Diego Zoo is undoubtedly a world class zoo. However, if you live in a city with a zoo, I would probably not visit the San Diego Zoo if vacationing here unless you really like pandas. Your zoo probably has 75% of the same animals and you probably don't care about the rest, unless they're pandas. If you do go and live in Southern California, buy the membership, it is reasonable and will save you money if you have guests.
There is no clear path to walk the entire zoo in a day, you have to double back at some point. Also the map only shows a select few of the animals, there are many more not shown. The aerial tram is useful to get from one side of park to the other, the far side generally has a long line in the afternoon. The bus tour can have a long wait if you don't catch it early, I wouldn't spend 30 minutes in line, you have to walk through most of it anyway.
