Both are large and require a lot of walking - the Wild Animal Park even more than the Zoo. You can easily spend an entire day at each one, but if you know what you want to see you can do quite a bit in just a few hours, too.
If I had to recommend one it would be the Zoo - there is a wider variety of animals there, and they have done a lot of interesting habitat exhibits like Tiger River, Absolutely Apes, Ituri Forest, Hippo Beach, Gorilla Tropics, etc. And you can also see the pandas and the koalas at the Zoo - can't do that at the Wild Animal Park.
But the Wild Animal Park (WAP) gives you a better opportunity to see the animals in open spaces out on a "savanna", similar to Animal Kingdom at WDW. Of course the problem is that sometimes you can't see the animals, or they are too far away. Still, they do have some good viewing areas for the savanna, and special viewing for the cheetahs and lions where you can usually see them.
If you don't want to walk a lot, then I think the bus tour at the Zoo gives you a better overlook of the entire Zoo than the tram ride does at the WAP.
As to how long it takes...depends on how fast you move and how long you look at things. I can make my way all around the Zoo in about 3 hours - but I don't stop to look at the birds or reptiles or other things that I don't find very interesting. That still gives me time to visit all of the big cats and the pandas and the bears, hippos, and polar bears, though.
For the WAP...it would take me closer to 4 hours, and that doesn't count doing the tram ride, which is a MUST do if you're going to visit the WAP.
Another big difference - the Zoo is fairly close to the coast whereas the WAP is inland - and it can get REALLY hot at the WAP, especially in the summer time. And a lot of the paths don't have that much shade.