The Wildlife park is really that small, huh? Perhaps I should consider adding something else onto that day's schedule. Thanks.
LAX
I have been to both parks in the last 12 months (actually about a dozen times but that is a different story).
Featherdale only makes sense when you have a lot of time or you combine it with a Blue Mountain tour. The advantage is that you can get real close to the Roos and the Koalas. Taking a picture with a Koala is free there. The park itself is quite small and can easily be done in a couple of hours.
Taronga Zoo is a big, "proper" Zoo. The Australian animals section ("Backyard to Bush") is quite nice and you can also walk in between the Kangaroos and one (!) Emu. However touching a Koala and having a picture taking will cost extra.
Taronga also has a very nice section to see a Platypus. Both in an Aquarium style environment as well as in a more natural state. And they are one of the most fascinating creatures this continent has to offer. In addition there is a very nice nocturnal area where you can see some exotic species.
The other thing to consider is that Featherdale is a boring bus ride out of the city while Taronga Zoo will start with one of the most spectacular ferry rides on the planet followed by a gondola ride to the entrance. Truly magical when the sun is out.
IMHO, if you just want to see Kangaroos and Koalas, then Fetherdale will do but should be combined with a Blue Mountain tour. Don't just go there for the park. Yes, Fetherdale has more animals but the main attraction is being close with the Roo and the Koala.
If you want to see more Australian animals than Taronga Zoo is the better place to be.
Food is bad at both places by the way.
Last but not least, google for coupons as there are frequently print out coupons offered on 3rd party sites which offer substantial discounts for both parks (same applies to Sydney Aquarium)