I am going to disagree with the majority here, I like both of them a lot, but if I had to only pick one, I would say the Wild Animal Park.
You are able to get closer to the animals.
There are a lot of things like the house with all of the Bonsai Trees, the desert environment, a Flower house, please be aware that some these only operate seasonly, but most are open ear around.
If you are going soon, the holidays have a lot more going on at the Wild Animal Park than at the zoo.
I will also say that if you are planning on going in the summer, despite all of this, I would recommend the Zoo, due to the hot weather.
Also, at both locations if you are disabled, they do have the option to take shuttles along the steepest parts of the parks for free, at the wild animal park this does mean that you may occasionally have to use one of the phones that are located throughout the park to call for assistance though, if you think you can make it up a hill and find out you can't, there is a number that you can call from a cell phone to get assistance, just stop in a guest relations and ask for an information packet on it.
By the way, I am lucky enough to have a season pass, so we get to go to both fairly often.