The best zoo-tycoon game in my opinion. The WP series features heavily on realism, unlike the Zoo Tycoon series, although that series is good too. Animals look realistic, as are their AI behavior. It takes a while for babies to be born, unlike ZT where it apparently happens instantly. Food don't magically appear out of thin air to feed your animals. like in ZT. You need to place the proper facilities to make the food.
WP 2 also focuses more on those rare/unknown animals, unlike ZT where they feature mostly well known/popular animals. I like this idea, because it lets people who play this game learn about animals they've never seen/or read about before. For example, there's the frilled sharks from the marine expansion. I doubt many people who aren't shark fans would know what they are.
The map isn't really set up like a grid (ZT again). You simply 'paint' the paths along the ground for your customers to walk on, which makes your zoo look more realistic if you're using stone or dirt paths for instance.
One thing I do like in ZT over WP2 though is setting up your habitats for marine animals. It's very easy to do in ZT but I find it frustrating to do the same in WP2.
Both zoo series are good, but if you want more realism in a zoo game, then WP2 is the way to go. With 150+ animals, realistic AI, along with loads of plants and attractions, etc, however, you should buy it anyway.