The hours might be be longer on the weekend, I am not sure though - The traffic will be lighter on the weekend. Although late on Sunday, the traffic will be tough as for all the people coming home from weekend aways to San Diego and Mexico.
The narrated tour is nice to take because it does take you all over the zoo and you can see everything in one tour. From that, you can decide what you thought looked cool and where you would like to start. The zoo is really big and there is a lot of walking. My only complaint about the San Diego zoo is that they make it look so much like the animals natural habitat, that sometimes it is hard to see the animals! It is a beautiful zoo and the landscape is unbelievable! You would really enjoy it!
The crowds will be lighter during the week, but it still won't be too bad on the weekend.
Have a great time!
Karen