For the WAP you need a solid 1/2 day. We might go to the beach in the morning, have lunch then head to the zoo around 2 or 3 and stay till closing. (in summer it closes at 8 and kicks you out by 9) We prefer this time of day as it does tend to get hotter at this park (more inland) and we like taking the tram when the sun sets as there are usually more animal moving about. As for the zoo, we like to go in the morning, take a break at lunch, then back for the afternoon and night. This would be more of a 3/4 day. Again, in the summer the zoo is open late and they have evening activities and shows planned. While the shows and activities are nice, for us the highlights are the animals and the zoo itself. Every year we go, I think why are we taking the kids back, I'm sort of sick of it. Then I get there and walk around and repeat many times, "Now I remember why I love this place". Also, weather at the zoo is cooler.
As for descriptions, the zoo is typical zoo-type though done much better than most zoo around the country (though I hear the zoo in DC is quite nice). The animal are in enclosures, the landscaping and layout is beautiful and pleasant to walk around (though hilly). The WAP is more spread out and the animals tend to be "loose" in larger "natural" settings. (Use the tram for viewing) They have shows that are entertaining (bird show has been the same for years, yet I still laugh at the emu jokes). I love the gorilla exhibit and lorakeets (you can buy food and they will land on you to eat ..... several at a time ........ great picture opportunity!)
If you can manage it, try to do both. This year we are only doing the zoo as only one is included in the city pass. For us to skip the WAP was an easy choice, but only because we have already done both a few times. Both have pros and cons. Sorry, this probably isn't much help.