I live about 10 mins from the Safari Park. I love that place. That said, I don't go there in the summer time unless it's already after 3 PM. I think the Zoo will be best for you. There are so many animals and things to see. I love the new stuff like the Australian Outback exhibit, Reptile Walk, and the Elephant Odyssey which is already a few years old still impresses me.
We had a chance to do a behind the scenes tour (not the Backstage Pass). We were lucky enough to win 4 passes. It was pretty awesome, so I would recommend that if you can afford it. We had a private tour, similar to the bus tour, went into the back of the lions/jaguars, fed a camel, fed the giraffes, and our guide was great.
We just got SW passes and went yesterday. It's fun, and there are more things to do in the summer, but if I were to choose between the zoo and SW, I'd def go with Zoo. Also, I like to think about where my money goes. When I spend money at the zoo, it goes back to the animals. Heck, my entire membership is a tax deduction!