The reason for that is that they are managed by the same zoological society.
there are different driving directions and maps available.
If you go, the zoo is built on several canyons, and have some serious hills (if you are pushing a stroller/wheelchair, but they do have a bus the drive around and oyu can see most of the exibits.
the Wild Animal park is a lot different, be prepared to walk a lot, as it is spread out!
both are great places to visit if you are on vacation, but each is a days worth!
Don't forget that if you have little ones, that
Lego Land is in the north county of San Diego.
(edit: just saw that you are not taking kids, forget lego land, just go to
Disneyland!! hehehe)